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Rock and Mineral Dictionary

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If the rock or mineral term you are looking for is not in the dictionary, please e-mail us and we'll add it.



Salt (NaCl or sodium chloride) is a crystalline chemical that occurs naturally around the world, in oceans and in salt deposits. The oceans on Earth contain salt (roughly 2.6% salt by weight), as do some lakes. The mineral halite, formed of NaCl, is a colorless to gray to brown crystal. Salt has a hardness of 2.5 and a specific gravity of 2.165. Its index of refraction is 1.5442. NaCl melts at 800.0 °C and boils at 1,465 °C. In solution (dissolved in water), salt is neutral (it is neither acidic nor basic).


Sand is a loose material composed of fine debris of rock and mineral particles (usually geology/rocks/glossary/indexq.shtml" target="_top">quartz) that are from 0.0625 to 2.0 millimeters in diameter.


Sandstone is a common type of grainy sedimentary rock that is made mostly of sand-sized grains (usually quartz) that are held together by silica, calcium carbonate, clay, or iron oxide.


Sapphire is a precious gemstone (a type of corundum) that ranges in color from blue to pink to yellow to green to white to purple (mauve sapphire) to pink-orange (padparadscha sapphire). Six-sided asterisms sometimes occur in star sapphires (caused by inclusions of tiny, thin, parallel needles of rutile). Sapphires are related to rubies. Sapphires were once thought to protect the wearer from poisonous creatures. Sapphire has a hardness of 9 and a specific gravity of 3.9 - 4.1. Sapphires are often heat treated to improve their color.

sapphire quartz

Sapphire quartz (also called blue quartz) is another name for blue chalcedony.


Sard is a semi-precious stone related to carnelian. This brownish-red, opaque gemstone was once used extensively for seals and was carved using intaglio. Sard was named for Sardis, the ancient capital of Lydia. Sardius is mentioned in the Bible, and may refer to jasper.


Sardonyx is a semi-precious stone that is formed by two layers, a red-brown layer of sard and a gray, white, black or brown layer of onyx. Sardonyx is a type of quartz. Sardonyx is frequently carved to make intricate cameos and seals.


A scarp (also called an escarpment) is a cliff or row of cliffs that were formed by formed by faults in the crust, erosion, or landslides.


Schiller (from the German term for play of colors or glitter) is an iridescent or bronze-like luster occuring in some minerals (it is also referred to as labradorescence when it occurs in feldspars). This optical effect is caused by submicroscopic lamella (thin layers or flakes of inclusions) contained within the mineral. These layers of inclusions can produce a bronze-like luster, golden iridescence, red color-play, and/or a blue-green sheen that flashes when viewed from certain angles. This type of inclusion can be valuable (as in sunstone and labradorite). Layers of shiller can block reflected light, decreasing the stone's brightness. Some types of feldspars (like labradorite, sunstone, spectrolite and peristerite) and other minerals (like hypersthene) exhibit schiller.


Schist is a type of crystalline metamorphic rock whose consistuent grains are in an organized pattern. Schist is formed from fine-grained sedimentary rocks like shale.


People study science to learn about the physical world.


Scoria are bomb-sized (greater than 64 mm across) pyroclastic fragments that are formed as blobs of lava cool when they are blasted through the air. Scoria are filled with cavities formed by trapped air bubbles.

seafloor spreading

Seafloor spreading is the movement of two oceanic plates away from each other, which results in the formation of new oceanic crust and a mid-ocean ridge.

sea glass

Sea glass (also called beach glass) is glass from old broken bottles, windows of wrecked ships, etc. that has been worn down and etched by the sea and sand over the years. This glass is smooth (with no sharp edges) and looks like beautifully sand-blasted glass with a soft patina. Pieces of this glass are collected on beaches and often made into jewelry items. Brown, deep green and clear are the most common colors of sea glass; after these come blue, amber and aqua. Rare colors include pink, red, purple, light yellow, and sea green.


A seamount is an underwater mountain that rises at least 50-100 m above the sea floor. Some seamounts rise above the water's surface. Most seamounts are volcanic in orgin; only a few are non-volcanic (caused by uplifting).


Sediment is loose mineral or biologic material that is deposited by water, ice, or air.

sedimentary rock

Sedimentary rock is rock that has formed from sediment, like sand, mud, small pieces of rocks. Over long periods of time, these small pieces of debris are compressed (squeezed) as they are buried under more and more layers of sediment that piles up on top of it. Eventually, they are compressed into sedimentary rock. It is made of strata, or layers. The layers that are farther down in the Earth are older than the top layers. Most fossils are found in sedimentary rock. sandstone, shale, and limestone are sedimentary rocks.


Seftonite is a translucent, moss green type of chalcedony. It is found in South Africa and North America.


Seismic means pertaining to earthquakes.


A seismigraph is a device that records and measures seismic waves (vibrations in the Earth), like those from earthquakes.


Selenite is a soft, colorless-to-slightly-colored, transparent mineral. It is a crystalline variety of gypsum. Selenite has a hardness of 1-2 and a specific gravity of 2.3 to 2.4.


Serpentine is a green stone made of silicate; there are two types of serpentine, bowenite and hydrated magnesium silicate. Bowenite is a jade-like stone (green to black) that is sometimes used in jewelry. The softer variety, hydrated magnesium silicate, is translucent serpentine has a hardness of 4 () - 5.5 (bowenite) and a specific gravity of 2.5 to 2.6. Serpentine is found in the British Isles and some other locations. Connemara marble (from Ireland) is a type of cloudy green serpentine.


Shale is a type of sedimentary rock that is formed from clay that has been pressed into thin sheets.

shield volcano

A shield volcano is a volcano that has gently sloping sides. Shield volcanoes are composed mostly of basalt (whose lava flows easily and quickly). The biggest volcanoes on Earth (like those on the islands of Hawaii) are shield volcanoes.
shocked quartz
Shocked quartz is quartz that has undergone deformation due to extreme pressure and heat. It has been found in the layer that marks the K-T boundary, leading credence to the Alvarez impact theory.

side vent

A side vent is an opening in the side of a volcano through which volcanic materials (like lava, gases, and pyroclastic debris) erupt.
Silica is a very common mineral composed of silicon and oxygen (SiO2). Silicates make up about 95% of the Earth's crust.
Silicates are minerals composed of silicon and oxygen with one or more other elements. Silicates make up about 95% of the Earth's crust.
Siliceous means made of or resembling silica.


Sillimanite (aluminum silicate, Al2 SiO5) is a transparent to translucent mineral that ranges in color from white to gray to brownish to greenish. Sillimanite has the same chemical formula as both kyanite and andalusite (these three minerals are polymorphs); Sillimanite is the rarest of the three (all three are used in high-temperature ceramics, used in spark plugs). Sillimanite has a hardness of 7.5 and a specific gravity of 3.2. Sillimanite is the state mineral of Delaware (since March 24, 1977); SIllimanite is found near near Wilmington, Delaware. Sillimanite is found in Brazil, New England (USA), and Europe; it is found in areas of high-grade metamorphic rock.
Silt is a particulate material made from loose bits of rock and/or mineral (sediment) that ranges in size from 0.002 to 0.0625 millimeters in diameter; silt is finer than sand, but coarser than clay. Silt is carried by moving water; when it is deposited, it is called sediment.


Siltation is the build-up of silt that is suspended in rivers or other bodies of water.
Siltstone is a fine-grained type of sandstone that is made from silt


Silver is a fine, silver-white metal often used in jewelry. Pure silver has a hardness of 2.5. Other metals are alloyed with silver (usually copper) for silver used in jewelry making. Silver tarnishes after exposure to air (a thin layer of silver-oxide forms on the surface). Silver often occurs near copper lodes.


The sky on Earth appears blue because our atmosphere scatters the blue colors from the sunlight (which consists of all colors of light). The sky on planets with a different atmospheric composition would appear to be a different color.

smoky quartz

Smoky quartz is a type of brownish quartz that has a smoky look.

snowflake obsidian

Snowflake obsidian (also called flowering obsidian) is a volcanic glass that is usually dark (black or brownish) with white "snowflakes". This glassy, lustrous mineral is found in lava flows, and obsidian stones can be massive. Obsidian is formed when viscous lava (from volcanos) cools rapidly. Most obsidian is 70 percent silica. Obsidian has a hardness of 5 and a specific gravity of 2.35.


Soapstone (also called steatite) is a soft, easily-carved, fine-grained metamorphic rock that can be green, brown, or gray. This stone has a greasy, soapy feel to it, hence its name. Soapstone is found worldwide. It is carved into figurines, beads, seals, bowls, pipes, cookware, and other items - it has been used since ancient times. Chemically, soapstone is composed mostly of talc, hydrated magnesium silicate (Mg3Si4 O10(OH)2) plus other minerals. Soapstone has a hardness of 1-1.5 (extremely soft - it can be scratched with a fingernail) and a specific gravity of 2.2-2.8.


Sodalite is a dark blue mineral with streaks of white, gray, pink, or green. It is used for carvings and jewelry. Sodalite is one of the mineral components of lapis lazuli. . Sodalite is sodium aluminum silicate chloride; its formula is Na4Al3(SiO4)3Cl. Sodalite has a hardness of 5.5 to 6 and a specific gravity of 2.1 - 2.3. Sodalite is found in Brazil, Canada, India, Italy, Namibia, United States, and Russia.


Soil is a natural, constantly-changing substance that is made up of minerals, organic materials, and living organisms. plants grow in soil.
Soil Layers diagram
soil horizons

Soil is made up of distinct horizontal layers; these layers are called horizons. They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers ( subsoil, regolith and bedrock).


A solid is one phase of matter, a phase in which the molecules are very close together and cannot move around.
South Magnetic Pole
The South Magnetic Pole is the point on the Southern Hemisphere of the Earth toward which a compass' needle always points; at the South Magnetic Pole, a compass' needle will stand vertically. It is now located just off the coast the continent of Antarctica (and its location changes over time).
South Pole
The South Pole is the point on the Southern Hemisphere of the Earth that is farthest north. It is 90° south of the equator.

specific gravity

The specific gravity (abbreviated s.g.) of a material is a comparison of its weight with the weight of an equal volume of water. Specific gravity measures the density of a material.


Spectrolite is another name for Finnish Labradorite (a variety of plagioclase feldspar). It is a fairly abundant grayish mineral that has brilliant iridescent flashes of color (usually green, blue, orange, or red) after it is polished. The crystals are transparent to translucent. Spectrolite is usually cut with a flat surface in order to highlight the brilliant flashes of color. Spectrolite has a hardness of 6 to 6.5 and a specific gravity of 2.70.


A spectroscope is an instrument that is used to identify gemstones. It works by determining the light waves that a stone absorbs; different stones absorb different wavelengths of light.


Speleology is the study of caves. A speleologist is a person who studies caves.


A speleothem is a mineral deposit formed in a cave. Speleothems are made when calcium carbonate (CaCO3) or other minerals precipitate (drip out of solution) from slow-moving cave water. Stalactites and stalagmites are examples of speleothems.


A spelunker is a person who explores caves.

Spessartine garnet

Spessartine garnet (also called Spessartite garnet) is a type of a garnet that is orange (it varies from reddish orange to brownish orange to yellowish orange). The chemical composition is Manganese Aluminum Silicate. Crystals vary from transparent to translucent. This relatively rare gem is found in Sri Lanka, Australia, Madagascar, Brazil, Sweden, Myanmar, and the U.S. Spessartine garnet has a hardness of 7.0 - 7.5, a specific gravity of 4.19 (relatively heavy), and a refractive index of 1.8.


Sphene (sometimes called titanite) is a mineral that comes in green, yellow, white, brown or black wedge-shaped crystals (sphene means wedge in Greek). Sphene is used only rarely as a gem (due to its relative softness). It's chemical formula is CaTiSiO5, Calcium Titanium Silicate. Sphene has a hardness of 5-5.5, a specific gravity of 3.3 - 3.6, and a white streak.


Spinel is a very hard semi-precious stone composed of octahedral crystals. Spinel ranges in color from red to black to yellow, frequently resembling rubies. Iron and chrome are components of spinel, giving it its color. Spinel belongs to the feldspar species and is found in in Burma, Sri Lanka and Thailand. Some varieties include: Balas ruby (red spinel), Almandine spinel (purple-red), Rubicelle (orange), Sapphire spinel (blue), Ghanospinel (blue), Chlorspinel (green). Spinel is also laboratory synthesized. Spinel has a hardness of 8, a specific gravity of 3.58-4.06, and a refractive index of 1.72.

spreading ridge

A spreading ridge is an area of the ocean floor in which new crust is being formed as magma erupts.

stabilized stones

Stabilized stones have been impregnated with plastic to improve its durability, stabilize cracks, and improve the stone's appearance (a dye is sometimes added to the plastic - this is called a color shot or color stabilized). Liquid plastic resins are injected into soft, porous stone at high pressures - the plastic fills the pores in the stone. Turquoise is often stabilized.


A stalactite is an icicle-shaped mineral deposit that hangs down from the roof (or upper wall) of a cave.


A stalagmite is a mineral deposit that grows upwards from the floor of a cave.

Star of Africa

The Star of Africa (also called the Cullinan diamond) is the largest diamond yet found, weighing 3,106 carats (roughly 1.3 pounds) in its rough form. It was mined at the Premier Mine in South Africa in 1905. This enormous gem was named for the chairman of the company that owned the mine. It was given to King Edward VII of England for his birthday in 1907. The diamond was cut (by Joseph J. Asscher of Amsterdam) into many stones, including the Cullinan I (530 carats, pendelique-brilliant shaped, the largest cut diamond in the world), the Cullinan II (317 carats, cushion shaped), Cullinan III (94 carats, pendelique shaped), Cullinan IV (63 carats, square-brilliant shaped), and many other smaller stones.

star garnet

A "star garnet" is almandine that exhibits an asterism. Almandine is a type of garnet that ranges in color from deep red to reddish-brown.

Star of India

The Star of India is a huge, blue, star sapphire weighing 563.35 carats. It is cut as a cabochon. This gemstone was found in Sri Lanka (date unknown). A British Army officer brought it to London, where it was cut by Albert Ramsay around 1905. It is now at American Museum of Natural History.

Star of the South

The Star of the South is a the largest diamond found in South America. This Brazilian stone weighs 254.5 carats.

star ruby

A "star ruby" is a ruby that exhibits an asterism, a six-pointed star of light (when cut as a cabochon). The world's biggest star ruby is the Rajaratna, which weighs 2,475 carats. The world's biggest double-star ruby (with a 12-pointed star) is the Neelanjali, weighing 1,370 carats. Most star rubies today are synthetic.

star sapphire

A star sapphire is a sapphire that exhibits an asterism in the form of a colorless, six-rayed star that reflects light. Star sapphires are cabochon cut. Laboratory-produced star sapphires ("Linde stars") were developed in 1947 by the Linde company; most star sapphires today are synthetic.

states of matter

Matter can exist in four states or phases (solid, liquid, gas, and plasma) and a few other extreme phases (like critical fluids and degenerate gases). The phase diagram of water (above) shows its phase at various temperatures and pressures.


Steatite (also called soapstone) is a soft, easily-carved, fine-grained metamorphic rock that can be green, brown, or gray. This stone has a greasy, soapy feel to it, hence its name. Soapstone is found worldwide. It is carved into figurines, beads, seals, bowls, pipes, cookware, and other items - it has been used since ancient times. Chemically, soapstone is composed mostly of talc, hydrated magnesium silicate (Mg3Si4 O10(OH)2) plus other minerals. Soapstone has a hardness of 1-1.5 (extremely soft - it can be scratched with a fingernail) and a specific gravity of 2.2-2.8.
Stishovite is a very dense form of quartz that has only been found in meteorite impact craters, in which quartz has undergone high-pressure shock.
Stratigraphy is a method of dating fossils by observing how deeply a fossil is buried. Sedimentary rock layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing them into rocks. Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed. Generally, deeper rocks and fossils are older than fossils found above them.


Stratocladistics is a method of classifying organisms based upon both cladistics (considering common ancestors with shared anatomical characteristics) together with stratiography (information from the fossil record which lets you know which animals lived earlier or later than others; older fossils are deeper than more recent fossils). In stratocladistics, cladograms are generated in which ancestors preceed their descendants. (see Science, 11th June, Vol 284, 1999)
A stratovolcano is a composite volcano.


The streak of a mineral is its color when it is in powdered form. You can determine the streak of a mineral easily by rubbing a rough mineral (not cut stones!) along the surface of a hard, unglazed porcelain sheet (this is called a streak plate - you can use the back of a white porcelain bathroom tile). Even though the color of a mineral may vary, the streak color is constant. Note: the streak plate has a hardness of about 6.6, so it can only test mineral that are less hard than that (since they must leave a streak on the porcelain). For example, the streak of chalcopyrite, graphite, magnetite, and pyrite is black, the streak of galena is gray, the streak of cinnabar, the streak of azurite and lapis is blue, the streak of malachite is green, the streak of turquoise is white with a green tint, the streak of olivine, amethyst, and tourmaline is white, and the streak of hematite is red-brown.


Striations are grooves, lines and scratches found naturally in some minerals.


A subduction is a phenomenon in which one part of the Earth's crust (a plate) is pushed underneath another plate as two plates collide. The descending crust melts as it is pushed deep into the Earth's mantle. Subduction destroys crust and recycles it back into the mantle.
subduction zone
A subduction zone is an area on a planet's crust in which the edge of an oceanic continental plate is being pushed beneath another plate.


Subliming is when a material goes directly from being a solid to being a gas (it skips the liquid phase altogether).

Suess, Eduard

Eduard Suess was an Austrian geologist who first realized that there had once been a land bridge between South America, Africa, India, Australia, and Antarctica. He named this large land mass Gondwanaland (named after a district in India where the fossil plant Glossopteris was found). This was the southern supercontinent formed after Pangaea broke up during the Jurassic period. He based his deductions upon the fossil fern Glossopteris, which is found throughout India, South America, southern Africa, Australia, and Antarctica.


Sugilite is a medium to dark purple semi-precious gemstone (it can also range from pink to brown to black). It is usually opaque with a waxy luster (but can be translucent) and often has brown, pink and white inclusions, looking like a purple version of turquoise. It is usually polished and not faceted. Sugilite has a hardness of 5.5-6.5 and a specific gravity of 2.75 - 2.80. This stone is not enhanced - massive stones are often found. Sugilite is Potassium Sodium Lithium Iron Manganese Aluminum Silicate; its formula is KNa2Li3(Fe, Mn, Al)2Si12O30. This stone was named for Ken-ichi Sugi, the Japanese geologist who discovered it in 1944. It is found in Iwagi Island, Shikoku, Japan and Mont Saint-Hilaire, Quebec, Canada, but the largest deposits are in northern South Africa.


Sunstone is also called aventurine feldspar (a variety of oligoclase). This gemstone varies from golden to orange to red-brown, and can be transparent or translucent. Sunstone is metallic-looking due to sparkling red, orange or green crystalline inclusions (these are hematite or goethite crystals). Sunstone is found in Canada, the USA (in Oregon), India, Norway, and Russia. This brittle stone has a hardness of 6 and a specific gravity of 2.63 - 2.67. Sunstone is not enhanced.


A supervolcano is an enormous volcano that is an order of magnitude larger than ordinary volcanoes. A supervolcano occurs when a huge magma chamber in the Earth's crust erupts after being under great pressure, causing a large caldera to form as the land over the magma chamber collaspses. This type of eruption is probably able to cause a major climate change and a mass extinction as it ejects dust, debris, and caustic gases into the atmosphere. Supervolcanos do not look like the familiar cone-shaped volcano - a supervolcano looks like a huge collapsed crater. Yellowstone National Park (in Wyoming and Montana, USA) is a supervolcano70 km long and 30 km wide; it is due to erupt soon (in terms of geological time).

Swiss lapis

Swiss lapis is not lapis lazuli at all. It is jasper dyed to resemble lapis lazuli and is misleadingly called "Swiss lapis."


Symmetry across an axis (also called bilateral symmetry) is when one side of an object is the mirror image of its other half - i.e., one half has the same shape and size as the other half (for example, most leaves are bilaterally symmetrical). Radial symmetry is when a basic shape is duplicated around a central point.

synthetic ruby

Synthetic rubies were first made by the French chemist Auguste Verneuil, who invented the flame-fusion process for producing inexpensive rubies in 1886. It was a ruby, but it didn't look much like one. In 1918, J. Czochralski invented the pulling method for growing inexpensive rubies. Carroll Chatham synthetic rubies (more expensive to produce, but natural-looking) were introduced in 1959. Kashan synthetic rubies were made beginning in 1979.

synthetic stone

Synthetic stones are made in laboratories; these stones generally lack imperfections. It is very difficult to distinguish a synthetic stone from a natural stone.
Rock and Mineral Dictionary

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If the rock or mineral term you are looking for is not in the dictionary, please e-mail us and we'll add it.

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