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Rock and Mineral Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.
If the rock or mineral term you are looking for is not in the dictionary, please e-mail us and we'll add it.

G



gabbro

Gabbro is a coarse, dark, intrusive, igneous rock. The crystals are large enough to see without a microscope. Chemically, gabbro is the same as basalt (gabbro forms when magma cools below the surface of the Earth, but basalt forms when the lava cools on the surface).


gagate

Gagate (popularly known as jet) is fossilized coal. It is a hard, lustrous black stone that was used in mourning jewelry during the Victorian era (especially after Queen Victoria's husband died and she went into a long-lasting mourning, affecting fashion). Jet is frequently cabochon cut. Gagate has been mined near Whitby (on the Yorkshire coast of England) since prehistoric times. It is also found in Spain. France, Germany, and Russia, but these other sources are said to be inferior to the harder, more elastic Whitby jet. Jet/gagate has a hardness of 2.5-4 (quite soft) and a specific gravity of 1.30-1.35 (it is relatively lightweight). Jet leaves a brown streak. When burnt with a red-hot needle, jet smells like coal Black glass and plastics are often used to imitate jet (glass is much heavier and harder than jet) - jet is warm to the touch.


gahnospinel

Gahnospinel is a rare blue spinel stone that is high in zinc and magnesium. It can only be distinguished from most spinel by its high specific gravity and high refractive index. Gahnospinel has a hardness of 8, a specific gravity of 3.97. Its chemical formula is (Mg, Zn)Al2O4.
garnet

Type of Garnet Color of Gem
Grossular Colorless, orange, yellow, pink, or brown
Pyrope Colorless, pink, or red
Pyrope-Almadine Red-orange to red-purple
Almandine-spessartine Red-orange
Chrome pyrope Orange-red
Almandine Orange-red to purple-red
Hessonite Yellow-orange to red
Spessartine Yellow-orange
Topazolite Yellow to orange-yellow
Malaia Yellow to red-orange to brown
Andradite Yellow-green to orange-yellow to black
Demantoid Green to yellow green andradite
Tsavorite Green to yellow-green
Pyrope-Spessartine Green-yellow to purple
Color-change garnet Blue green in sunlight; purple-red in incandescent light
Transvaal "jade" Bright green grossular garnet
Uvarovite Emerald green
Grape purple
Rhodolite Purple-red
Xalostocite Pink grossular garnet

garnet

Garnets are any of a group of semi-precious silicate stones that range in color from red to green (garnets occur in all colors but blue). Some garnets used as gemstones include pyrope (the deep red garnet), almandine, spessartine, grossular, the iron-aluminum dark red garnet (also known as the carbuncle stone), Uvarovite (rare), and the lustrous Andradite (which includes the valuable green demantoid garnet, Topazolite , and Melanite). Red garnet is the birthstone for January. Garnet has a hardness of 6-8 and a specific gravity of 3.5 - 4.3. The formula for garnet is: (Mg, Fe, Ca or Mn) with Al2Si3O12.

gas

Gas is a phase of matter in which the molecules are widely separated, move around freely, and move at high speeds.


gaspeite

Gaspeite is a pale green to apple-green semi-precious gemstone that often has brown inclusions of its host rock. Gaspeite is translucent to opaque. This beautiful stone has only recently been used in jewelry, and is often set in silver. Gaspeite has a hardness of 4.5 - 5, and a specific gravity of 3.7. Gaspeite is Nickel Magnesium Iron Carbonate; its chemical formula is (Ni, Mg, Fe)CO3. This stone is found in Gaspe Peninsula, Quebec, Canada (where it was originally found and from which it derives its name) and Kambalda and Widgie Mooltha, Western Australia, Australia.


gemstone

A gemstone (also called a precious stone) is a mineral that is valuable, rare and often beautiful. A few organic materials, like amber, coral and pearls are also considered gemstones.


geode

A geode is a rock whose crystal-filled interior can be hollow or filled. The crystals that form within the mineral crust of the geode is called druze. From the outside, geodes often look like rounded, but otherwise ordinary rocks.

geological time

The history of the earth is described in geological time, which is measured in millions of years and billions of years. The divisions used are: eon, era, period, and epoch.
geologic map
A geologic map is a map that notes the structure and composition of geologic features, like the presence of minerals, rock types, earthquake faults, underground water, and landslide areas.


geology

Geology is the study of the physical and chemical structure of the Earth, and its history, including studying rocks and minerals.

geologist

A geologist is a scientist who studies geology.


geomorphology

Geomorphology is the scientific field that investigates how landforms are formed on the Earth (and other planets).


GIA

GIA stands for the Gemological Institute of America.


girasol

Girasol (which means sunflower in Spanish) is a yellow or orange type of precious opal. Girasol is also known as hyacinth opal. In girasol, the play of colors seems to come from within the stone, like a floating light, and seems to follow the light source.
glacier
glacier

A glacier is a large, long-lasting mass of ice and snow that moves very slowly over the land like a slow-moving frozen river. Glacial ice is fresh water (not salty); glaciers contain the largest reservoir of fresh water on Earth.

glacier outburst flood

A glacier outburst flood is a sudden flood caused as water flows quickly from a glacier or a glacier-dammed lake. It is caused as volcanic activity under the glacier melts the ice.

gneiss

Gneiss is a coarse-grained type of metamorphic rock that usually has light and dark bands (the bands are formed by the recrystallization of quartz, feldspars, micas and amphiboles).
Karat Percent Gold
24 Kt. 100% Gold
18 Kt. 75% Gold
14 Kt. 58.3% Gold
10 Kt. 41.7%

gold

Gold is a precious metal that is very soft when pure (24 Kt.). Gold is the most malleable (hammerable) and ductile (able to be made into wire) metal. Gold is alloyed (mixed with other metals, usually silver and copper) to make it less expensive and harder. The purity of gold jewelry is measured in karats. Some countries hallmark gold with a three-digit number that indicates the parts per thousand of gold. In this system, "750" means 750/1000 gold (equal to 18K); "500" means 500/1000 gold (equal to 12K). Alloyed gold comes in many colors:
Colored Gold Alloy Metal(s) Added to the Gold
Yellow Gold 50% silver and 50% copper
White Gold Nickel, zinc, copper, tin and manganese
Pink (rose) Gold 90% copper and 10% silver
Green Gold High proportion of silver or cadmium
Blue Gold Some iron
Grey Gold 15-20% iron

goldstone

Goldstone (also known as aventurine) is a shimmering quartz stone that ranges in color from yellow to red to light green to light brown. The shimmer is caused by tiny metallic particles (mica) within the stone (not gold).
Gondwana

Gondwanaland

Gondwanaland (also known as Gondwana) was the southern supercontinent formed after Pangaea broke up during the Jurassic period. It included what are now South America, Africa, India, Australia, and Antarctica.


goshenite

Goshenite is the pure, colorless form of beryl (Be3AlSiO6, related to emerald and aquamarine). This hard, transparent gemstone is named for the town of Goshen, Massachusetts, where it was first found. Goshenite has been found in North and South America (especially Colombia), Northern Europe, East Africa, South Africa, and the Himalayan mountains in Asia. Goshenite has a hardness of 7.5 - 8.0 and a specific gravity of 2.6 - 2.8. It is not enhanced. Goshenite is sometimes coated with a green foil to resemble an emerald.

graben

A graben (or rift) is a valley between two faults.


grain

A grain is a unit of weight used for diamonds and natural pearls. Four grains are equal to one carat.

granite

Granite is a type of igneous rock (volcanic rock that has cooled) that is light-colored and coarse grained; it is composed of at least 65% silica. Granite is characterized by the minerals orthoclase and quartz with some plagioclase feldspar and iron-magnesium minerals. Granite underlies much of the continents.


granitic magma

Granitic magma is a type of molten rock from which granite is formed.


Grape garnet

Grape garnets are a rare, intense violet to purple-red garnet. Grape garnets are made up of almandite and spessartite. They have a hardness of 7-7.5 and a specific gravity of 3.8 - 3.9. Grape garnets are found in the Orissa district of northwestern India.


gravel

Gravel is unconsolidated bits of rock fragments that are bigger than 2 millimeters across.


green diamond

Green 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.


green garnet

Green garnets are Demantoid garnets, a valuable green, and very lustrous type of garnet. They are a rare variety of andradite. Demantoid garnets have characteristic inclusions that look like horsetails. Demantoid garnets has a hardness of 6-7 and a specific gravity of 3.8 - 3.9. Demantoids were very popular in the 1800's, but are rarely used today.


greenhouse effect

The greenhouse effect is an increase in the temperature of a planet as heat energy from sunlight is trapped in the atmosphere. Excess carbon dioxide and water vapor increase this effect. The greenhouse effect is strong on Earth and Venus, maintaining warm temperatures.


greenstone

Greenstone is another name for nephrite, a semi-precious stone and a variety of jade. Nephrite is slightly softer that jadeite and is often veined; it is used in carvings, for making beautiful bowls and vases.


griqualandite

Griqualandite is tiger's eye from Griqualand, South Africa. It is a yellowish-brown to reddish-brown gemstone that has a silky luster. This gemstone has bands of yellow and brown; when viewed from the opposite direction, the colors are reversed. Tiger's eye is usually highly polished and set as a cabochon (or cut as a bead) to display the stone's chatoyancy (light reflected in thin bands within the stone). Tiger's eye is a type of chatoyant quartz with fibrous inclusions (especially crocidolite). This stone is sometimes heat-treated. Tiger's eye has a hardness of 7.0.


grossular garnet

Grossular garnet is a type of garnet, calcium-aluminum silicate. Hessionite is a transparent brown, yellow, orange, or honey-colored variety of grossular garnet often used in jewelry. The yellow variety is called cinnamon stone, hyacinth or jacinth. Transvaal "jade" is a type of green to gray grossular garnet from South Africa. Pink grossular garnets varieties include landerite, rosolite, andXalostocite. Tsavorite is an emerald-green grossular garnet. Grossular garnet has a hardness of 6-7.5 and a specific gravity of 3.6.


Gutenberg discontinuity

The Gutenberg discontinuity separates the outer core and the mantle of the Earth.


gutta percha

Gutta percha is a resin from the Isonandra gutta tree. Jewelry was made from gutta percha in the mid-1800's. Gutta percha was also used to insulate electrical cables. The Gutta percha company was founded by Dr. Montgomerie in 1845 and was in business until 1930.


gypsum

Gypsum is a sedimentary rock that is formed by the chemical precipitation of calcium, sulfur, and oxygen.

EnchantedLearning.com
Rock and Mineral Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.
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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