EnchantedLearning.com Rock and Mineral Dictionary

maar A maar is a shallow, flat-bottomed crater that forms above a diatreme (volcanic vent) when steam explodes. Maars range in size from 200 to 6,500 feet in diameter and from 30 to 650 feet deep. Maars often fill with water and form lakes.

mafic Mafic refers to dark-colored igneous rock or magma that have significant amounts of iron and magnesium. For example, olivine, augite, and hypersthene are mafic rocks.

magma Magma is molten rock (lava) from which igneous rock forms. Magma is be formed from many types of rocks, including basalt, andesite, dacite, gabbro, and rhyolite

magma chamber A magma chamber is an underground cavity filled with gas-rich magma (molten rock). This magma erupts through volcanoes.

magnetic field The Earth's magnetic field is aligned with the north and south poles, and has reversed many times during geologic history. Sharks may use the magnetic field of the Earth for navigation purposes on migrations.

magnetite Magnetite is a black to gray opaque magnetic mineral composed of iron and oxygen (it is magnetic iron ore). Magnetite is found worldwide. Magnetite, Fe3+2Fe2+O4, has a hardness of 5.5-6 and a specific gravity of 5.1 - 5.2. The streak is black.

mahogany obsidian Mahogany Obsidian (also called Apache tears) is brownish obsidian. 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. The pin above is Mahogany (brown) obsidian.

malachite Malachite is an opaque semi-precious stone with layers of deep green and light green. It is usually found in copper mines; malachite is about 57% copper. Malachite was used as jewelry thousands of years ago by the ancients Egyptians. Malachite is usually cabochon set in silver. Russian malachite carvings were done in miniature and in large scale; malachite was also inlaid in furniture. Malachite has a hardness of 4 and a specific gravity of 3.80. It is monoclinic; it has one two-fold axis of symmetry. Malachite is sometimes coated with colorless wax, oil, or hardening agents to increase its durability and enhance its appearance.

malleable Malleable metals are easily worked with a hammer or a roller. gold and sterling silver are very malleable metals.

mantle The mantle is the layer of the Earth located between the crust and the molten core.

marble Marble is a metamorphic rock.

marcasite Marcasite is a shiny, metallic semi-precious stone. It is actually iron pyrite. Marcasite is generally faceted. The Czech pin above is studded with marcasites.

mass extinction Mass extinction is the process in which huge numbers of species die out suddenly. The dinosaurs (and many other species) went extinct during the K-T extinction, which was probably caused by an asteroid colliding with the Earth.

maw-sit-sit Maw-sit-sit is a rare green gemstone that has dark-green to black veining; it sometimes has white spots. The stone is opaque to translucent. Maw-sit-sit has a hardness of 6.0; the specific gravity is 2.5 to 3.5. This stone is found in Maw-sit-sit, Myanmar (Burma), and is often found neat jadeite, but maw-sit-sit is not a type of jade. Maw-sit-sit was first identified in 1963 by the Swiss Gemologist Eduard Gübelin; the local called the stone maw-sit-sit, and the name was retained. Although its exact composition is still unknown, Maw-sit-sit is composed of chromite, ureyite, chrome jadeite, symplektite, chrome amphibole, and other lighter minerals.

melee A melee is a small diamond, under .20 carat.

mesa A mesa is a land formation that has a flat area onthe top and steep walls - mesas usually occur in dry areas.

Mesozoic Era The Mesozoic Era ("The Age of Reptiles"), occurred from 245-65 million years ago. It is divided into the Triassic, Jurassic, and Cretaceous periods. Dinosaurs, mammals, and flowering plants evolved during the Mesozoic, and Pangaea broke up. The era ended with the K-T mass extinction.

metamorphic rock Metamorphic rocks are compacted by pressure and heat from deep inside the earth.

meteor A meteor is a meteoroid that has entered the Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star. Most burn up before hitting the Earth.

meteorite A meteorite is a meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.

meteoroid Meteoroids are tiny stones or pieces of metal that travel through space.

Mexican diamond Mexican diamond is a misleading term for rock crystal, and not a diamond at all.

Mexican jade Mexican jade is a misleading term for dyed stalagmitic calcite; it is not jade.

milk and honey effect The milk and honey effect is one in which the apparent coloring of a stone changes (from milky to the color of honey) as the angle of the light changes. When a light is shined on the stone, one side of the stone is the color of milk and the other is the color of honey. When the light changes direction, the color effect also shifts. Cat's-eye chrysoberyl can exhibit this milk and honey phenomenon.

1,000,000 million A million is a thousand thousand. The dinosaurs lived millions of years ago.

mineral A mineral is a naturally-occurring solid of definite chemical composition whose atoms usually form a regular pattern. About 4,000 different types of minerals have been identified. Each mineral has a unique chemical makeup and structure, and can be identified by its physical, structural, and optical properties (like its location, color, luster, streak, hardness, specific gravity, translucency and refraction of light).

Mohorovicic discontinuity The Mohorovicic discontinuity separates the crust and the upper mantle of the Earth.

Substance Hardness Talc 1 Amber, Fingernail, Ivory, Shell, Jet 2.5 Gold 2-3 Bronze, Coral, Pearl 3 Iron 4 Glass 5 Opal 5.5-6.5 Amethyst, Chalcedony, Quartz, Steel (pocket knife) 7 Spinel, Topaz 8 Ruby, Sapphire 9 Diamond 10 Mohs scale The Mohs Scale of Hardness measures a substance's hardness, that is, how resistant it is to being scratched. In the Mohs scale, which ranges from 1 to 10, one substance is harder than another if it can scratch it. For example, a diamond (hardness = 10) will scratch garnet (hardness = 6.5-7.5), but not the other way around, so a diamond is harder than garnet. This scale was invented by Austrian mineralogist Friedrich Mohs (1773-1839).

monogenetic volcano A monogenetic volcano is one that was formed during a relatively short time period and during a single eruption. Most monogenetic volcanos are basaltic.

Montana ruby A Montana ruby is actually a pyrope garnet (and not a ruby at all).

moonstone Moonstone (orthoclase) is a semi-translucent stone that is made of albite and orthoclase feldspar. It is usually whitish-blue, but can be colorless, yellow, orange, gray, or even reddish. Moonstone is usually set as a cabochon. Moonstone was very popular early in the 20th century and was extensively used in Art Nouveau jewelry. Moonstone has a hardness of 6 and a specific gravity of 2.57. It is monoclinic; it has one two-fold axis of symmetry. Adularia is a common type of moonstone. Oligoclase is another type of moonstone; Labradorite and albite are rare forms.

morganite Morganite is a transparent to translucent pink gemstone. It is a variety of beryl, Be3Al2(Si6O18) that contains some manganese (giving the stone its pink color). Morganite has a hardness of 7.5 - 8 and a specific gravity of 2.71 - 2.90. It has poor cleavage and is brittle. Morganite is often heat treated to give the stone a purer pink color (and remove any yellow spots). Morganite was named for J. Pierpoint Morgan, the American industrialist and gem collector. Morganite is found in Brazil, Madagascar, Italy, Pakistan, Mozambique, Namibia, Zimbabwe, and the USA (California, Maine, Connecticut, and North Carolina).

moss agate Moss agate (also called Mocha stone in Britain and the USA) is a green variety of agate. It is chalcedony that has dendritic (tree-like) inclusions of green (red or black) hornblende. The inclusions often form beautiful patterns. Moss agate has a hardness of 6.5-7 and a specific gravity of 2.6. Green moss agate is found in India, and some other locations.

mother-of-pearl Mother-of-pearl is the iridescent coating on the inside of oyster shells. Mother-of-pearl is used for jewelry, buttons, and other uses.

moukaite Moukaite is a semi-precious gemstone, a variety of jasper from Australia. The color of moukaite varies from red-browns to pinks, with gold and white bands.

mountain A mountain is a very tall high, natural place on Earth - higher than a hill. The tallest mountain on Earth is Mt. Everest.

mudflow A mudflow (also called a debris flow or lahar) is a moving mixture of rock, water, and other debris.

1,000,000 mya "mya" stands for millions of years ago.

mya yay Mya yay is the Burmese (Myanmar) name for the highest quality translucent jadeite.

mystic fire Mystic fire (also called mystic topaz or rainbow topaz) is topaz 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.

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