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2- Durability
A high degree of durability in precious gemstones is necessary as this is the only factor that allows the beauty of a gemstone to be long-lasting, also only a durable gemstone can take and retain a good polish. Hardness is a measure of durability based on mineral’s ability to resist scratching and abrasion. Reaumur was the first one who proposed a method to measure the hardness of a mineral in 1722. He investigated the hardness a substance as the results of pressing together the edges of two right-angled prisms made of the substances to be compared. Later in 1822, a more practical means of assessing hardness was proposed by a German geologist Friedrich Mohs. He used ten species of minerals as standards of increasing hardness. These minerals in order of increasing Mohs hardness are 1) Talc 2) Gypsum 3) Calcite 4) Fluorite 5) Apatite 6) Orthoclase 7) Quartz 8) Topaz 9) Corundum 10) Diamond. This means that Gypsum can scratch Talc, Calcite can scratch Gypsum and so on. The number 7 on Mohs’s scale is an important one, as any gemstone must be at least as hard as this if it is to withstand the action of grit in the air or on clothes.
Crystals tend to break or split parallel to certain definite directions. This property is called cleavage of a crystal and it is one of the consequences of those regular arrangements of the atoms which distinguish crystals from amorphous solids. Cleavage is described as perfect, distinct or indistinct. Cleavage and fracture (atomic structure) affect the strength of a gemstone and the ease with which it can be worked. Extra care must be taken when working with gemstones that have one or more directions of perfect cleavage. It is worth noting that where the break is not related to the atomic structure (fracture) the resulting surface will not be flat or smooth.
Crystals tend to break or split parallel to certain definite directions. This property is called cleavage of a crystal and it is one of the consequences of those regular arrangements of the atoms which distinguish crystals from amorphous solids. Cleavage is described as perfect, distinct or indistinct. Cleavage and fracture (atomic structure) affect the strength of a gemstone and the ease with which it can be worked. Extra care must be taken when working with gemstones that have one or more directions of perfect cleavage. It is worth noting that where the break is not related to the atomic structure (fracture) the resulting surface will not be flat or smooth.
3- Scarcity
In addition of being beautiful and durable, for a gemstone to be classified as a precious gemstone, it should be scarce. However scarcity itself does not guarantee that a gemstone will be a precious gemstone. It is largely an issue of supply and demand; the more that the demand outstrips the supply, the scarcer a gemstone becomes.
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