amorph
A mineral is amorphous if it shows the same physical behaviour in all directions. There is internal order of the atoms and molecules. Amorphous material can alter its internal structure and become crystalline, but this is uncommon (Opal --> Chalcedony).

Asterism
Asterism refers to the presence of a star-shaped light reflection on the surface of gem rough cut as cabochons. The source of this effect is the presence of inclusions of foreign minerals which reflect the light in a star-shaped way. Rubies and sapphires, cut as cabochons, usually show 6-ray stars indicative of their symmetry, and sometimes 12-ray ones. Other gemstones, such as garnets, may have 4-ray stars. If the reflecting inclusions are not fully symmetric, the stars are distorted or uncomplete which decreases the price of a gemstone. The narrower and sharper the rays, the greater the value, if  color and transparency are equal. Synthetic gemstones can also show asterism.

Cats-eye effect
A cat's eye refers to a light effect showing a single band of bright light. Similar to asterism, this effect is caused by included minerals or small hollow channels which reflect the light. Only cabochons show this effect. If a cabochon is perfectly cut, the cats-eye "wanders" across the surface of the stone. This effect appears with many gemstones including chrysoberyl, elbaite, etc. Similar effects are also called falcon-eye or tiger-eye. Chrysoberyl shows the best cats-eye.

ct (carat)
The international measure for the weight of gemstones is a carat (ct). 1 ct is equivalent to 0.2 grams.
(ct / diameter) relation for diamonds (density approximately 3.5):
0.10 ct = 3.0 mm
0.25 ct = 4.1 mm
0.50 ct = 5.2 mm
1.00 ct = 6.5 mm
2.00 ct = 8.2 mm
The term carat is also used for the standard of gold and gold alloys but is not a weight measure. 1 ct is 1/24 of the metal weight. Therefore pure gold has 24 ct.

Cleaveage
Crystals with "cleaveage" cleave or split in certain directions due to their crystal structure.  If crystals  have no cleavage, they simply break. There are typically microscopic cracks or cleaves that can be seen under magnification. The gemstone cutter always has to consider the cleavage mechanism of a mineral. Topaz and apatite have strong cleavage planes, while quartz has none. Cleavage increases the difficulty of cutting.

Color
Gem colors can be caused by the mineral's intrinsic color, impurities in the part per million level, foreign inclusions, or crystal structure. One can distinguish between intrinsic coloring and other sources using various tests. The "streak check" can determine if the color is intrinsic (malachite remains green while sapphire is white.) The "streak check" is done by scratching the mineral on a rough porcelain plate.

Crystal
A crystal is a chemically and physically homogeneous solid which is terminated by plain and smooth sides, and which has a symmetric internal structure. There are seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. The crystal's planes often are not in a typical proportion but the angles between the analogue planes are always the same (legitimacy of constant angles).

Density ("specific gravity")
Density is the quotient of mass and volume of a material. Units of measurement for density are typically grams per cubic centimeter or g / cm³. Differences in density help one identify unknown materials and allow one to estimate the weight of stones based on size. The basis of all comparisons is pure water at 4° Celsius which has a density that is internationally defined as 1 g / cm³ at standard conditions.  Specific gravity is a ratio of the density of material compared to water. Each material with a specific gravity over 1 is  heavier than water. 

Deposit
Concentrations of gemstones are typically found in a specific area (e.g. mountains or pegmatites). Local occurences are called deposits. One can distinguish between primary and secondary deposits. At primary deposits the mineral is still at its original location. At secondary deposits the mineral has been transported to a new location, e.g. washed away by a river or glacier. At the primary deposit the mineral is still within the original rock, and its crystals in original condition. Secondary deposits usually contain rounded (by a river) crystals or crystal fragments. Rivers can transport minerals many kilometers. Because of the higher density many minerals deposit at special locations in a river so that rich, narrow limitated deposits occur. This applies also for gold deposits ("nuggets"). The most famous example for washed ashore deposits are the diamonds found on the coast of Namibia. Water-borne secondary deposits are known as alluvial deposits.
Gemstone deposits are irregularly distributed on earth. Some rich focal points are the Rocky Mountains (USA), Minas Gerais (Brazil), Ural mountains (Russia), Myanmar, south and east Africa, and possibly Antarctica.

Dispersion
When white light passes through a mineral, it can be dispersed into its spectrum colors ("dispersion"). A good cut can cause a dispersion and refraction of the light so that a crystal shows the fully sparkling spectrum of colors.  In diamonds, this is refered to as "fire". Zircon, rutile, and demantoid show an excellent dispersion similar to that of diamond.

Gemology
Scientifically dealing with gemstones.

Gemstone
A mineral (also organic material as pearls, corals and amber), distinguished by the combination of as many as possible of these properties: Great lustre, transparency, beautiful color, hardness, strong dispersion, resistance, rarity, and sufficient size of the crystals for the production of jewelry.
The value of gems also depends on the mode and country-specific preferences. Gemstones appear in different mineral groups.

Hardness
Durability of a mineral against penetrating of a material into it. Amongst the international gemology society the Mohs (German gemologist, 1773-1839) scale is widely used which compares 10 minerals, depending on their ability which scratches which. The Mohs scale reaches from talcum (hardness 1) to diamond (hardness 10). Because the hardness of minerals doesn't increase regularly, other scientists have invented different scales (see our page Mohs Scale) which considers the depth of penetration.

Inclusion
An inclusion is a foreign element within a crystal, mostly gas, fluid or another mineral. Synthetic crystals usually don't contain any inclusions. Inclusions usually reduce the value of a gemstone, depending on the visibility by the naked eye or a loupe (10 times). Sometimes inclusions are an indication on the locality, e.g. the "horse-tail" inclusions in Russian demantoids which even increase their value in comparison with demantoids from other locations. Inclusions can also cause sought light effects like asterism.

Labradorescence
Shimmering, metallic colors which appear especially on the surface of the minerals labradorite and spectrolite. Blue and green colors are frequent. Stones which show the full spectrum colors are very sought.

Luminescence
Appearance of a mineral in ultraviolet light. If one irradiates certain minerals with ultraviolet light in the darkness, they partionally flash in different colors. If this strange gleaming is continuing for a while, after switching-off the lamp, it is called phosphorescence. Depending on the deposit, the same mineral shows luminescence or not (e.g. hauyne), depending on the content of foreign metals or trace elements. Some minerals glow when heated ("thermoluminescence", e.g. fluorite).
For gemological inspections only two shortwave ranges play a role: shortwave uv light (2,800 - 2,000 Å), and longwave uv light (4,000 - 3,150 Å). Fluorescent inspection helps detecting forgeries. For inspections of pearls x-rays are used which show typical luminescence of culture pearls, natural pearls, sea water pearls and fresh water pearls.

Lustre
Lustre depends on the smoothness of a crystal's or a cutted gemstone's surface. The more light is reflected the more lustrous a stone appears. Ultimate lustre is called "diamond lustre". Depending on the mineral one uses the expressions "glass lustre" (topaz), "metal lustre" (pyrite), "mother-of-pearl lustre" (labradorite), "silk lustre" and "resin lustre" (amber) etc.

Mineral
Anorganic material or chemical element which can be defined by a special chemical formula and crystal structure. Minerals often contain inclusions of gases, liquids or other minerals which show the natural origin (crystal growth).

Mineral Class
Minerals are usually classified according to their chemical composition and crystal structure. Today the classicication after Hugo Strunz is widely used. Strunz invented 9 mineral classes:
1. elements
2. sulfides
3. halogenides
4. oxides and hydroxides
5. nitrates, carbonates, and borates
6. sulfates, chromates, molybdates, and wolframates
7. phosphates, arsenates, and vanadates
8. silicates
9. organic compounds

Opalescence
Milky-blueish gleam of a mineral (e.g. common opal), caused by the reflection of mainly blue light.

Opalizing
Multicolored and limited colors on the surface of opals which change depending on the angle of view resp. "wander" across the surface. Ideally all colors of the spectrum appear. Reason for this effect are very small balls of the mineral cristobalite which reflect the light in interference colors ("rainbow colors").

Pleochroism
If a mineral is lightened from different angles, and shows different colors, it is named pleochroism. Depending on the crystal structure there are different types of pleochroism: dichroism (two colors, e.g. tourmaline), trichroism (three colors, e.g. tanzanite), and pleochroism (more than three colors).
Pleochroism can appear from weak to strong. The gem cutter has to consider the pleochroism so that the finished stone shows the desired brightness and color.

Refraction
Light is not only reflected by the surface of a crystal (see "lustre") but also penetrates a mineral. Depending on the mineral the light is more or less deflected. The intensity of deflection is determined by the refraction index. There are different refraction indices within a crystal, depending on the crystal system. Amorph crystals have just one refraction index. The measurement of the refraction index is an important aid regarding the differentiation between natural and synthetic gemstones.

Synthetics and imitations (history)
2000 b.c.: Egypts produced imitations of gemstones by using glass and enamel.
1758: Joseph Strasser (Vienna) created a sort of glass which could be cutted. It looked similar to diamonds ("Strass-Stones").
Around 1830: First synthetic gemstones.
Around 1890: A.V. Verneuil created a method for production of synthetic rubies ("Melting drops method").
1910: A.V. Verneuil started with the production of synthetic sapphires, later with corundum (ruby and sapphire) in different colors.
1926: Production of synthetic spinels with the Verneuil method.
1947: First synthetic star rubies and star sapphires in the USA.
Around 1947: Synthetic emeralds.
1948: Synthetic rutiles.
1955: First synthesis of diamonds in the USA and Sweden.
1970: Diamond synthesis in gem quality.
1977: Creation of an artificial gemstone (yttriumcirconoxide), called "Circonia".
Artificial gemstones have to be declared as artificial if traded.

Transparency
Minerals let the light pass through differently. The value of a gemstone usually increases with its transparency. One distinguishes between transparent, translucent, and opaque.

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