Minerals, Crystals and Gems
Born of fluid, heat, and pressure, minerals dazzle us with their breathtaking colors and shapes and astonish us with their usefulness. They are forged underground, where forces that have been at work for billions of years continue to make more minerals.
Every crystal has an orderly, internal pattern of atoms, with a distinctive way of locking new atoms into that pattern to repeat it again and again. The shape of the resulting crystal-such as a cube (like salt) or a six-sided form (like a snowflake)-mirrors the internal arrangement of the atoms.
As crystals grow, differences in temperature and chemical composition cause fascinating variations. In order to readily show their geometric form and flat surfaces, crystals need ideal growing conditions and room to grow. When many different crystals grow near each other, they mesh together to form a conglomerated mass. This is the case with most rocks, such as granite mentioned above, which is made up of many tiny mineral crystals.
The internal arrangement of atoms determines all the minerals' chemical and physical properties, including color. Light interacts with different atoms to create different colors. Many minerals are colorless in their pure state; however, impurities of the atomic structure cause color. Quartz, for example, is normally colorless, but occurs in a range of colors from pink to brown to the deep purple of amethyst, depending on the number and type of impurities in its structure. In its colorless state, quartz resembles ice.
Scientists typically describe crystals as "growing," even though they are not alive.
In subterranean gardens, they branch and bristle as trillions of atoms connect in regular three-dimensional patterns. Each crystal starts small and grows as more atoms are added. Many grow from water rich in dissolved minerals, but they also grow from melted rock and even vapor. Under the influence of different temperatures and pressures, atoms combine in an amazing array of crystal shapes. It is this variety and perfection of form and symmetry that has long drawn scientists to the study of minerals. Symmetry is a regular, repeated pattern of component parts. In crystals, these repeated patterns occur within the basic atomic structure and reflect the pattern of faces of the crystal.
Certain crystals are cut into gems because of their extraordinary color, whilst others such as diamonds for their hardness and brightness and flashes of color (known as "fire"). In general, any attractive, relatively flawless crystal can be cut into a gem. Although almost any of the four thousand kinds of minerals could be cut into a gemstone, in practice most gems used in jewelry today come from only about fifteen different mineral-crystals.
Size, beauty, rarity, and durability are the basic criteria that determine a gem's value. Because of their beauty and the skill and artistry necessary to cut gemstones from crystals, gems have always been expensive.
One factor in the value of a gem is its weight. Gem weight is measured in carats; one metric carat equals 0.2 grams (.007 ounces).
The word carat comes from carob, a Mediterranean tree whose seed was for centuries the standard for weighing precious stones.
Gemstones are described by gemologists using technical specifications. Eg, what is it made of, or its chemical composition. Diamonds for example are made of carbon (C), rubies of aluminum oxide (Al2O3).
Gems are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.
Gems have refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster.
They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.
Material or flaws within a stone may be present as inclusions. The gem may occur in certain locations, called the "occurrence."
A gemstone is prized especially for great beauty or perfection so appearance is almost always the most important attribute of gemstones. Characteristics that make a stone beautiful or desirable are color, unusual optical phenomena within the stone, an interesting inclusion such as a fossil, rarity, and sometimes the form of the natural crystal. Diamond is prized highly as a gemstone since it is the hardest naturally occurring substance known and is able to reflect light with fire and sparkle when faceted.
There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include:
Agate
Alexandrite and other varieties of chrysoberyl
Amethyst (originally a "cardinal gem", but now no longer so, since huge quantities were discovered in Brazil and the price plummeted)
Aquamarine and other varieties of beryl
Chrysocolla
Chrysoprase
Diamond
Emerald
Feldspar (moonstone)
Garnet
Hematite
Jade - jadeite and nephrite
Jasper
Kunzite
Lapis lazuli
Malachite
Obsidian
Olivine (Peridot)
Opal (Girasol)
Pyrite
Quartz and its varieties, such as tiger's-eye, citrine, agate, and amethyst
Ruby
Sapphire
Spinel
Sugilite
Tanzanite and other varieties of zoisite
Topaz
Turquoise
Tourmaline
Zircon
Minerals that infrequently occur in gem quality form:
Andalusite
Axinite
Benitoite
Bixbite (Red beryl)
Cassiterite
Clinohumite
Iolite
Kornerupine
Natural moissanite
Zeolite (Thomsonite)
There are a number of organic materials used as gems, including:
Amber
Bone
Coral
Ivory
Jet (lignite)
Mother of pearl
Ammolite - from fossils formed from the shells of extinct ammonites.
Pearl.
Metals ores found in the earth, like gold and silver, are minerals.
Crystalline substances, like salt and quartz, are minerals.
Homogeneous natural substances, like water and gas, are also minerals.
These non-solid minerals, however, are not considered "rock."
The word rock refers to solid mineral deposits.
Rocks are minerals, but not all minerals are rocks.
"The Royalty of the Mineral Kingdom"
Generally speaking, a gemstone is a stone that is beautiful, rare, and durable (resistant to abrasion, fracturing and chemical reactions).
Some minerals can be very beautiful, but they may be too soft and will scratch easily (such as the mineral fluorite).
Others are too common and are given a semi-precious status (such as agate).
All 'gemstones' have good hardness and a high index of refraction / great sparkle.
Most gems are silicates which can be very stable, hard minerals.
A few gems are oxides and only one gem, diamond, is composed of a single element, carbon.
There are also a few gemstones that are not true minerals (called mineraloids) but are included here: opal, amber, and moldavite.
Gemstones in the Elements class:
DIAMOND
MOISSANITE
CORUNDUM
Variety: RUBY
Variety: SAPPHIRE
CHRYSOBERYL
Variety: ALEXANDRITE
Variety: YELLOW-GREEN CHRYSOBERYL
Variety: CAT'S EYE CHRYSOBERYL
PERICLASE
SPINEL
TAAFFEITE
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| Gemstones in
the Carbonate class: • RHODOCHROSITE
• SINHALITE (a borate mineral) |
Gemstones in the Silicate class: • ANDALUSITE • AXINITE • BENITOITE • BERYL Variety: AQUAMARINE Variety: EMERALD Variety: GOSHENITE Variety: HELIODOR Variety: MORGANITE Variety: RED BERYL • CORDIERITE Variety: IOLITE • DIOPSIDE • EUCLASE • THE GARNETS: ALMANDINE ANDRADITE Variety: DEMANTOID GROSSULAR Variety: HESSONITE Variety: TSAVORITE PYROPE SPESSARTINE UVAROVITE • JADE (a variety of two minerals: Jadeite and Nephrite) • KORNERUPINE • OLIVINE Variety: PERIDOT • QUARTZ Variety: AMETHYST Variety: CITRINE Variety: ROCK CRYSTAL Variety: ROSE QUARTZ Variety: SMOKY QUARTZ • PECTOLITE Variety: LARIMAR • SCAPOLITE • SPODUMENE Variety: KUNZITE Variety: HIDDENITE • TITANITE (also known as sphene) • TOPAZ • THE TOURMALINES: ELBAITE Variety: INDICOLITE • ZIRCON • ZOISITE Variety: TANZANITE Gemstones in the Mineraloids class : • AMBER • OPAL • TEKTITES Variety: MOLDAVITE |
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Gemstones in the Phosphate class: • APATITE  •
BRAZILIANITE |
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