EnchantedLearning.com Rock and Mineral Dictionary

radioisotope dating Radioisotope dating is used to find out how old fossils are. Different radioisotopes are used for different geologic time periods because of their different half-lives.	rainbow calsilica Rainbow calsilica is a newly-found, multi-colored, layered stone composed of calcium and silica. This stone has been recently used for Zuni fetish carvings and in some jewelry (beads and cabochon cut stones). Rainbow calsilica was only recently found in Mexico or Northern South America (it's origin remains mysterious). Some people theorize that this stone formed as a result of the runoff of mining or oil-drilling chemicals, and has only formed in the last 30 to 50 years (but this is uncertain).	rainbow obsidian Rainbow obsidian is another name for obsidian that is iridescent.
rainbow opal Rainbow opal is a type of precious opal that has curved bands of colors (that resemble rainbows).	rainbow topaz Rainbow topaz (also called mystic topaz or mystic fire) is enhanced that has been color enhanced by coating it with a fine layer of metal atoms (in a process called vacuum deposition). This stone has red, green, violet, and blue streaks. Mystic fire has a hardness of 8.	Rajaratna The Rajaratna is the biggest-known "star ruby" (a ruby that exhibits an asterism, a six-pointed star of light, cut as a cabochon). It weighs 2,475 carats.
rati The rati is an Indian unit of weight that is used for gemstones. A rati is equal to 0.91 carats. The rati has varied in the past (and in different regions).	Raviratna The Raviratna is the biggest-known ruby. It weighs 3,600 carats.	reconstructed stone A reconstructed stone is one that is made from pieces of smaller stones or crystals). Reconstructed stones often have telltale air bubbles. For example, "Geneva rubies" (reconstructed rubies) are made from tiny ruby crystals that have been fused together. This type of stone is generally no longer manufactured (except reconstructed amber, which is stilll made) because synthetic stones are vastly superior to reconstructed stones.
red beryl Red beryl is a is a rare, deep red variety of beryl. Gemstone-quality forms of this mineral are found in only one place in the world, in the Wah Wah Mountains, near Beaver, Utah, USA. Small crystals of this gem were first found in 1905 in the Thomas Range in Juab County, Utah. The mine bearing gemstone-quality red-beryl was found in the 1950's. The biggest red beryl crystal ever found was 14mm by 34mm, weighing about 54 carats. The average faceted red beryl gemstone weighs about 0.15 carats. Red beryl has a hardness of 7 - 8, a specific gravity of 2.66-2.70, and a refractive index of 1.564-1.574. The chemical formula Be3Al2SiO6, with other trace elements. Internal flaws in beryl gems can be hidden by treating the stone with oil (this is often not disclosed to the buyer).	red diamond Red diamonds are rare, fancy diamonds and are quite valuable. Diamonds are precious, lustrous gemstones made of highly-compressed carbon; they are one of the hardest materials known. Diamonds have a hardness of 10, a specific gravity of 3.5, and a refractive index of 2.417 - 2.419.	refraction When light enters a medium with a different optical density (like a gemstone), the light is bent at an angle (and also changes its speed); this is refraction.
refractive index The refractive index is a measure of how light is refracted in a substance (like a gemstone). In doubly-refractive stones, the light is split into two light rays when it enters the stone, and the rays travel in different paths - so these stones have more than one refractive index.	refractometer A refractometer is a device that measures the refractive index of a gemstone.	relief map A relief map is a topographic map that uses different colors or shades to indicate elevations.
repose Repose is the time between volcanic eruptions.	reticulite Reticulite (also called thread-lace scoria) is a very light form of pumice. It is formed during very high fountaining volcanic eruptions, when a lot of air is included in the rock	rhodium Rhodium is a white precious metal. Rhodium is extremely expensive and is often used to plate precious and base metals, giving jewelry a hard, platinum-like sheen.
rhodochrosite Rhodochrosite is a mineral whose color ranges from rose to pink to almost yellow or brown. Although it is very pretty, this stone is soft and brittle; it is used in jewelry and for carvings and figurines. Rhodochrosite is Manganese Carbonate; its chemical formula is MnCO3. Rhodochrosite has a hardness of 3.5 - 4.5 (glass has a hardness of 4) and a specific gravity of 3.5. Rhodochrosite is found in Argentina, Peru, Germany, Mexico, South Africa, Russia, Italy, USA (Colorado and Montana), and Romania. Rhodochrosite is not enhanced.	rhodolite Rhodolite (meaning "rose stone" in Greek) is a purple-red to pink-red variety of garnet. It is a combination of almandine and pyrope (it is sometimes called pyrope-almandine garnet). This silicate stone has a hardness of 7-7.5 and a specific gravity of 3.5 - 4.3. The formula for garnet is: A3B2(SiO4)3. Rhodolite is found in the US, Zimbabwe, Kenya, and Sri Lanka (Ceylon). Rhodolite is not enhanced.	R Horizon The unweathered rock (bedrock) layer that is beneath all the other layers.
rhyolite Rhyolite is a a type of volcanic lava or rock that is usually light in color; it contains 69 percent silica and is high in potassium and sodium.	Richter, Charles F. Charles Francis Richter (April 26, 1900- April 30, 1985) was a who developed the Richter scale, a logarithmic scale that measures the intensity of an earthquake. He developed it in 1935 at the California Institute of Technology.	Richter Scale The Richter scale is a logarithmic scale that measures the intensity of an earthquake. It was developed in 1935 by Charles F. Richter. The magnitude of an earthquake is calculated from the logarithm of the amplitude of waves recorded by seismographs. Beno Gutenberg also contributed to the more general application of the Richter scale. A magnitude 2.0 or less earthquake is called a microearthquake and is not felt by people. A magnitude 4.5 or more earthquake can be measured by seismographs all over the world.
rift A rift (or graben) is a valley between two faults.	ring of fire The ring of fire is an area around the Pacific Ocean that is high in volcanic, mountain-building, and seismic activity.	riverstone Riverstones are smooth, rounded pebbles found in rivers and on beaches. The action of the water and other rocks on riverstones polishes them naturally. Riverstones can be used as beads/stones in jewelry.
rock A rock is a aggregation of solid matter, a random conglomerate of minerals. The earth's crust is made of rock. There are three types of rock: igneous, sedimentary, and metamorphic. Petrology is the scientific study of rocks.	rock crystal Rock crystal is a transparent, crystalline mineral. Rock crystal is the purest form of quartz and a semi-precious stone.	rock cycle The rock cycle is decribes the relationship between igneous, sedimentary, and metamorphic rocks. James Hutton (1727-1797) first developed the concept of the rock cycle.
Rocky Mountain ruby A Rocky Mountain ruby is actually a pyrope garnet (and not a ruby at all).	rose de France Rose de France is a recent name for a very pale variety of amethyst. It is also known as lavender amethyst. Rose de France has a hardness of 7.0. This pale lilac transparent gem is found worldwide and is a type of quartz. Rose de France is sometimes heat-treated in order to lighten its color and/or to remove smokiness.	rose quartz Rose quartz is a form of quartz that ranges in color from pink to deep red.
rough Rough stones or crystals are in their natural state, they are neither cut nor polished.	rubellite Rubellite (sometimes spelled rubelite) is a red variety of tourmaline. Rubellite is red in both incandescent light and daylight, and is more valuable than other varieties of red tourmaline. Rubellite has a hardness of 7-7.5 and a specific gravity of 3.02-3.25. Rubellite is sometimes treated with fillers to increase the clarity of the stone.	ruby Rubies are precious stones and a member of the corundum family (Al2O3). Rubies range in color from the classic deep red to pink to purple to brown. Rubies are extremely hard; only diamonds are harder. During the renaissance, people thought that rubies could counteract poison. Laboratory-produced rubies were created in the 1890's; they are difficult to distinguish from natural rubies. The biggest ruby in the word is the Raviratna, which weighs 3,600 carats. Rubies have a hardness of 9 and a specific gravity of 3.9 - 4.1. Rubies are found in Afghanistan, Australia, Brazil, Cambodia, India, Myanmar (Burma), Malagasy Republic, Malawi, Pakistan, Sri Lanka (Ceylon), Tanzania, Thailand, United States (Montana and North Carolina), and Zimbabwe (Rhodesia).
ruby spinel A ruby spinel (or spinel ruby) is deep red, transparent spinel (not a ruby).	Rumanite Rumanite is a type of opal that is from Romania.	runoff Runoff is water (or other liquids) that drains or flows from the land into streams and rivers, eventually into seas. The water is generally from rain or snowpack melt.
rutilated quartz Rutilated quartz is a type of rock crystal which contains long, fine needles of rutile crystals (titanium dioxide). This beautiful stone is usually cut as a cabochon. It is also known as Venus' Hair Stone, Cupid's darts, and Fleches d'amour (arrows of love).

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