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Classification of precious gems

The name "gemstones" is highly conditional. Stones were named so for the opportunity to use them in jewelry. The exact number of stones used in jewelry not be named. This list is constantly being updated - life, fashion and technological progress always update it. The way people view the value of a gem is changing. Some appreciate it for its "healing" properties, other stone would bring "happiness" (according to a legend), the third like the durability of stones, and a fourth just invest in them. One thing remains constant - the beauty. It is the cause that allows one stone forever to be in the "status" of jewelry, and other (including synthetic gems) rightfully take their place in jewelry.

The concepts of "precious" and "semi-precious" became outdated and controversial; concepts ‘jewelry", "jewelry-ornamental " and "ornamental" that most accurately reflect the use of interesting stones are introduced. But interest of people for beautiful stone remains the same over years.

Strictly speaking, in mineralogy the term "stone" does not exist - the land is made up of minerals and rocks. Minerals are natural solid formations of the earth's crust, uniform by chemical composition and mostly crystalline in structure.

Rock Is a natural mineral compound. Bodies can be loose (sand), consolidated (clay mineral or sandstones) and solid (obsidian, jasper, granite).

Gemstones can be minerals as well as rocks. In addition, from ancient times to the present day are being considered as jewelry pearls, coral, amber – the materials of organic origin that do not belong to minerals and rocks. Numerous synthetic materials continue list of jewelry.

Hardness - resistance to scratching .

Hardness of minerals is usually determined using the Mohs scale. Mohs scale is a comparative set of minerals of varying hardness, where hardness of diamond (10) is accepted as the maximum unit.

Hardness of tested stone is checked by scratching it with reference "hardness pencil". Reference pencil is a mineral built into the metal handle with the number corresponding to its hardness. A complete set of pencils provides a check on the hardness of any mineral. Standard hardness pencils are designed for rough diamonds and are not suitable for jewelry, as jewelry deal with cut stone. For this purpose, you need to have a pencil having at the end a fragment of mineral with no more than 2.5 mm in cross section. Testing for hardness of cut stone is held only in those cases where other diagnostic methods have failed. Do not test hardness on the face of cut stone. Hardness checking should be started from pencil with lower hardness up to those with higher. And in any case, do not try using diamond pencil (№ 10) - it scratches everything, so it will give no result (except for the diamonds).

Brittleness (frailty) is the ability of stone to crumble under pressure. This property must be considered when planting stone in the seat and tacking it.

Viscosity - resistance to impact. Cases of unusual viscosity are specially regarded and are considered in the choice of material for the masonry work.

Fracture – characteristics of fragment’s surface. It is an important diagnostic feature. For jewelers it may be useful in the diagnosis even of cut stones, by looking at chips through a magnifying glass. Fractures can be conchoidal, splintery, fibrous, smooth, rough, chunky and earthy.
Color of the gem is one of the most important diagnostic features. Color can be idiochromatic, allochromatic and pseudocromatic.

Idiochromatic - proper color, associated with the elements of own composition, stated in chemical formula.

Allochromatic - extraneous color, caused by the presence of finely dispersed mechanical admixtures (agate, jasper, aventurine, etc.).

Pseudocromatic color is caused by the emergence of color effects due to reflection and scattering of light (cat's eye, star, effect of moon rocks, etc.).
Transparency (light transmission) is the ability of a mineral to transmit light. Transparency is determined by property of the material to transmit light and is characterized by the terms: transparent, translucent and opaque.
A transparent material is called one through which you can clearly see other objects.

Translucent material is the one through which you can not clearly see objects, but only their vague general features.

Opaque material is the one that does not transmit light. Sometimes are used such terms as translucent and semi translucent.

Refraction is the deviation of the light beam direction at the entry into a different medium. It is characterized by refractive-index, i.e. ratio of the speed of light in air to the speed of light in a stone. The refractive index is measured by a special device - refractometer. The refractive index is one of the most important features of diagnosis. Refractive index of gems varies from 1.434 (in fluorite) to 3.14 (for hematite). The refractive index of diamond is 2.418. The higher the refractive index of the stone, the greater is its brilliance or lucter.

Luster is the appearance of the mineral in reflected light. Luster can be glass, diamond, semi metallic, and metallic. Practice showed that glass shine stones have a refractive index in the range of 1.3 ... 1.9 (most of gems); stones with diamond luster have index 1.9 ... 2.6; semimetallic - 2.6 ... 3, 0, metalllic - more than 3.0. Luster also depends on the nature of the mineral surface and is defined as fatty, waxy, silky, pearly, opaque, and resinous.