Minerals / Gemstones crystal Systems

 

Crystal Structure

The crystal structure of a gem material determines how the gemstone absorbs light. Crystal shape may also influence the shape of a finished gemstone: most gemstones are cut to maximize the yield from the rough stone. Familiarization with crystal systems will give you more insight about why gemstones each have unique properties. Most gem materials are formed when a mineral crystallizes deep in the earth. The shape of the crystal is a function of the mineral's chemical formula and the way the atoms are bonded together and arranged in a precise pattern. Generally transparent gemstones have been cut from a crystal. There are seven basic crystal systems:

• Cubic
• Tetragonal
• Orthorhombic
• Monoclinic
• Triclinic
• Hexagonal
• Trigonal

 

Isometric System (Cubic System)

The isometric system is comprised of three equal length axes that are all at right angles to each other. This system is also called the cubic system. Think of a cube of sugar. Gemstones that crystallize in the cubic system include diamond, garnet, spinel and fluorite.

Cubic system

Diamond

Diamond is a transparent gemstone composed of carbon atoms arranged in a crystal lattice structure. Diamond is the hardest mineral on the Mohs scale with a rating of 10.

Natural and synthetic diamond crystal shape 

Garnet

Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Garnets possess similar physical properties and crystal forms, but differ in chemical composition.

Spinel

Spinel is a magnesium aluminum oxide mineral that occurs as octahedral crystals, flattened crystals, and twins. Spinel is often found in ruby and sapphire deposits.

Fluorite

Fluorite is a halide mineral composed of calcium and fluorine. Fluorite occurs in a variety of colors, including purple, green, yellow, blue, and white. The purple and green colors of fluorite are the most popular for gemstones.

Tetragonal System
The tetragonal system has three axes that all intersect at right angles; but two are equal in length and the third can be longer or shorter. Zircons  crystallize in the tetragonal system.

 

 

 

 

Hexagonal System
The hexagonal system has four axes; the three axes on the horizontal plane intersect at 60^o angles. The fourth axis intersects the horizontal plane at right angles. Gem material with the hexagonal crystal shape are easily identifiable crystals with six-fold symmetry. Gemstone materials that form in the hexagonal system include: corundum, Beryl, Tourmaline and Quartz .

 

Hexagonal System 

Trigonal System

Some minerals of the hexagonal system form with a three fold symmetry rather than six; these can be called "trigonal"; but the trigonal system is a subgroup of the hexagonal system.

 

 

Orthorhombic System
The orthorhombic system has all three axes at right angles, but all axes are a different length. Gemstones that crystallize in orthorhombic crystals include Topaz, Peridot, Andalusite and Chrysoberyl 

 

Orthorombic System 

Monoclinic System
The monoclinic system has two axes at right angles and one that is not perpendicular. All axes are different lengths. Gem materials with monoclinic crystal systems include jadeite, nephrite, malachite and azurite.

 

Monoclinic System 

 

Triclinic System
The triclinic system has three axes that are different lengths; none of the axes is at right angles. Gems in the triclinic system include turquoise and plagioclase feldspar (labradorite).

Triclinic System 

Cryptocrystalline
Some minerals form in clusters of microscopic crystals; these gemstones are called "cryptocrystalline". Cryptocrystalline materials will usually be translucent to opaque rather than transparent. Chalcedony  is a cryptocrystalline form of  quartz; varieties of chalcedony include agates, chrysoprase and bloodstone 

 

Amorphous
Some gem materials are not crystalline but amorphous in structure; opal and obsidian are common examples. Amorphous materials have no definite internal structure.

 

Organic Gem Materials
Organic gem materials are derived from living forms or are by-products of living things. Coral  is formed from the skeletal material of a small animal that forms hard structures that branch like a plant.Amber  is a fossil resin of ancient pine trees that lived about 30 million years ago. Ivory is taken from the tusks of elephants or walruses.Pearls  form around irritants that invade various mollusks.

CRYSTAL SYSTEMS

Cubic
Three crystal axes of equal length intersect at right angles to each other. e.g. diamond, spinel, garnets.

 

Tetragonal

Three axes intersect at right angles to each other. The vertical axis is of unequal length while the two horizontal axes are of equal length. e.g. zircon, rutile.

 

Hexagonal
Four crystal axes. Three are of equal length and intersect at 60o to form a horizontal plane which the fourth intersects at right angles. The vertical fourth is of unequal length and forms an axis of 6-fold symmetry. e.g. Beryl, apatite.

 

Trigonal
Four crystal axes. Three of equal length intersecting to form a horizontal plane which is intersected at right angles by the fourth axis. The vertical fourth is of unequal length and forms an axis of 3-fold symmetry. e.g. quartz, corundum, tourmaline, dioptase, haematite.

 

Orthorhombic (Rhombic)
Three crystal axes of unequal length interest each other at right angles. e.g. topaz, peridot, Chysoberyl, iolite, sinhalite, andalusite.

 

Monoclinic
Three axes. Two of unequal length intersect each other obliquely to form a plane which is intersected by the vertical third (of unequal length) at right angles. e.g. jadeite, nephrite, diopside, orthoclase feldspar, serpentine, sphene, malachite, spodumene.

 

Triclinic
Three axes of unequal length intersect each other at oblique angles. e.g. turquoise, labradorite.

angles of crystal system

    CRYSTAL SYSTEM SYMMETRY

Singly Refractive: Amorphous -- no crystal structure 

 

			Optic Axis
Cubic	9 planes	4 3-fold	-
	13 axes	3 4-fold	-
	a centre	6 2-fold	-

Doubly Refractive:

			Optic Axis
Tetragonal	5 planes	1 4-fold	uniaxial
	5 axes	4 2-fold
	a centre

 

			Optic Axis
Hexagonal	7 planes	6 2-fold	uniaxial
	7 axes	1 6-fold
	a centre

 

			Optic Axis
Trigonal	3 planes	3 2-fold	uniaxial
	4 axes	1 3-fold
	a centre

 

			Optic Axis
Orthorhombic	3 planes	3 2-fold	biaxial
	a centre

 

			Optic Axis
Monoclinic	1 axis		biaxial
	a centre

 

			Optic Axis
Triclinic	no planes		biaxial
	no axes
	a centre

 

Uniaxial
The optic axis of the crystal is parallel to the main crystal axis. One direction of single refraction.

 

Biaxial

There are two directions of single refraction. (optic axes)

 

GEMSTONES BY CRYSTAL SYSTEM (major ones)

*Diamond simulants, man-made                                        (U) = uniaxial, (B) = biaxial

 

Cubic	Tetragonal(U)	Hexagonal	Trigonal (U)
Diamond Sodalite Fluorite Spinel GGG * Strontium Titanate* Garnet Yttrium Aluminate* Lazurite (Lapis Lazuli) Yttrium oxide* Pyrites Cubic Zirconia*	Apophyllite Idocrase Rutile Scapolite Wardite Zircon	Apatite Aquamarine Beryl Goshenite Morganite	Calcite (marble) Quartz Corundum Rhodochrosite Dioptase Tourmaline Hematite
Orthorhombic (B)	Monoclinic (B)	Triclinic (B)	Amorphous
Andalusite Marcasite Chrysoberyl Peridot Danburite Sinhalite Enstatite Staurolite Iolite Topaz Kornerupine Zoisite	Azurite Nephrite Diopside Orthoclase Feldspar Epidote Serpentine Euclase Sphene Jadeite Spodumene Malachite	Axinite Kyanite Microcline Feldspar Plagioclase Feldspar Rhodonite Turquoise	Amber Chrysocolla Glass Ivory Jet Moldavite Obscidian Opal