Advertisement.

EnchantedLearning.com is a user-supported site.
As a bonus, site members have access to a banner-ad-free version of the site, with print-friendly pages.
Click here to learn more.

ad
(Already a member? Click here.)

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.

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

M



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.
Obsidian

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.


mica

Mica is a soft, lightweight, transparent mineral that cleaves into thin, elastic sheets (it has a single perfect cleavage direction). It is used for lampshades and electrical insulators. There are about 30 different types of micas, ranging in color from yellow to green to gray to violet to white to brown. Mica has a hardness of 2.5 and a specific density of about 3. Tiny mica particles give the shimmer to aventurine (goldstone).


microcrystalline

Microcrystalline is a type of mineral structure in which the crystals are so small that they can only be seen using a microscope.


micron

A micron is a unit of length equal to 0.001 mm (millimeter).


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).
Miocene epoch
The Miocene epoch was a geologic time span that lasted from 24 million to 5 million years ago. During the Miocene, modern birds and many mammals evolved (including the horses, dogs, bears, South American monkeys, and apes in southern Europe). Ramapithecus appeared; this primate was pehaps an ancestor of man. The climate was relatively warm (as compared to the earlier Oligocene or the later Pliocene). There were extensive grasslands (and grasses diversified), and the interior of the continents were relatively dry; northern coniferous forests expanded. Kelp forests appeared in the seas. In North America, the Sierra Nevada Mountains and the Cascade Mountains were formed; in the South America, the Andes Mountains arose. The Bering land bridge formed during the Miocene, connecting Russia and Alaska, allowing many animals to migrat across continents. The Isthmus of Panama also formed during this time. Australia became very dry (but went through wet and dry periods). In Europe, the Mediterranean Sea and Indian Ocean were separated (they had been connected by the Tethys Sea).


mogul emeralds

These Indian emeralds were owned by Indian moguls (like Shah Jahan, the builder of the Taj Mahal) and were inscribed with sacred blessings.


Mohorovicic, Andrija

Andrija Mohorovicic (1857 - 1936) was a Yugoslavian geophysicist. After examining seismic waves from the 1909 Kulpa Valley earthquake, Mohorovicic theorized that a boundary between the Earth's crust and the upper mantle existed (about 50 km beneath the surface) in which the speed of earthquake waves became very rapid. This region is now called the Mohorovicic discontinuity. A crater on the dark side of the moon was also named for Mohorovicic.


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).


Moissanite

Moissanite is a very hard mineral that was discovered by Dr. Ferdinand Henri Moissan (1852-1907), a French chemist and Nobel Prize winner (Moissan did work on synthesizing diamonds and discovered carborundum in 1891). He found tiny amounts of Moissanite in the iron meteorite that was found at Diablo Canyon (also called Meteor Crater) in Arizona, USA. Moissanite ranges in color from colorless to blue to green to yellow. Its chemical makeup is Silicon Carbide (SiC); it is also called Carborundum. Moisannite crystals are transparent to translucent. Moissanite has a hardness of 9.25 (this is almost as hard as diamond) and a specific gravity of 3.1 - 3.2. Laboratory-grown Moissanite is sold as a gemstone.


moldavite

Moldavite is a rare, glassy, translucent, dark green gemstone. Moldavite is a silica-based tektite, a mineral formed when a meteorite (a rock from space) struck the Earth's surface and melted and fused the surrounding rock. Moldavite is only found in Bohemia (the Czech Republic) in the Ries Crater in the Moldau River valley (which it was named for). Moldavite was discovered in the late 1800's; the meteorite from which it formed hit the Earth about 14.7 million years ago. Moldavite has a hardness of 5.5-6.6. Inclusions of gas bubbles and iron/nickel spherules are common. This natural glass has been used for jewelry, religious articles, and decorative objects since prehistoric times.


monoclinic

Monoclinic minerals have a crystalline structure in which there is one two-fold axis of symmetry. Jade, Malachite, orthoclase, and moonstone are monoclinic.


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.

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.

Enchanted Learning®
Over 35,000 Web Pages
Sample Pages for Prospective Subscribers, or click below

Overview of Site
What's New
Enchanted Learning Home
Monthly Activity Calendar
Books to Print
Site Index

K-3
Crafts
K-3 Themes
Little Explorers
Picture dictionary
PreK/K Activities
Rebus Rhymes
Stories
Writing
Cloze Activities
Essay Topics
Newspaper
Writing Activities
Parts of Speech

Fiction
The Test of Time
iPhone app
TapQuiz Maps - free iPhone Geography Game

Biology
Animal Printouts
Biology Label Printouts
Biomes
Birds
Butterflies
Dinosaurs
Food Chain
Human Anatomy
Mammals
Plants
Rainforests
Sharks
Whales
Physical Sciences: K-12
Astronomy
The Earth
Geology
Hurricanes
Landforms
Oceans
Tsunami
Volcano
Languages
Dutch
French
German
Italian
Japanese (Romaji)
Portuguese
Spanish
Swedish
Geography/History
Explorers
Flags
Geography
Inventors
US History

Other Topics
Art and Artists
Calendars
College Finder
Crafts
Graphic Organizers
Label Me! Printouts
Math
Music
Word Wheels

Click to read our Privacy Policy

E-mail


Enchanted Learning Search

Search the Enchanted Learning website for:



Advertisement.



Advertisement.



Advertisement.


Copyright ©2003 EnchantedLearning.com ------ How to cite a web page