PHYSICAL PROPERTIES OF GEMSTONES

The manner in which the atoms of a gemstone are arranged and the strength of the bonds between them directly affects the physical properties of gem stones, their durability (hardness, toughness, and stability), the way they break or cleave and their relative density (specific gravity).

The ability of a gemstone to withstand general day-to-day wear and tear sufficiently to be mounted in a piece of jewellery and to keep its polish without becoming unduly scratched, cracked or worn is important and will affect its value. Generally speaking, the harder the gemstone the better it can take and retain a good polish.

Hardness

In gemmology, hardness is a measure of how easily a surface can withstand abrasion caused by wear and tear including scratching. If jewellery items containing a range of different gemstones are stored together, the harder gems such as ruby and sapphire will scratch the less hard pieces, for example emerald, amethyst, or opal.

Because hardness is related to the crystal structure and the strength and direction of the atomic bonds, the gemstone may be scratched more easily in one direction than another. This differential hardness is particularly important for the lapidary when assessing how best to cut and polish a gemstone and in choosing a suitable mount or setting for the piece.

MOHS' SCALE OF HARDNESS

The relative degree of 'scratch ability' or susceptibility to scratching can be assessed using the Mohs' scale of hardness. In 1822 Friedrich Mohs, a German mineralogist took ten minerals that he was able to obtain easily and tested them against each other. Mohs arranged the minerals in order, from the softest (talc) that could be scratched by all the others, to the hardest which could not be scratched (diamond). He then assigned them numbers from the softest (1 = talc) to the hardest (10 = diamond).

The test is a comparative one, and the scale is not linear and does not increase by equal increments, for example the difference in hard­ness between 1 (talc) and 9 (the corundum group, which includes ruby and sapphire) is less than that between 9 and 10 (diamond).

Other hardness scales include: a) the Brinell Scale, which measures how much of a dent can be made when a steel ball is pushed into the surface (this cannot be used on particularly brittle, thin or fragile pieces, as they would break);

b) the Knoop scale which uses a diamond to make a measurable dent; and

c) the Vickers scale which also uses a diamond to make a dent.

The Mohs' test for hardness is quicker, cheaper and easier than the other methods. An estimate of hardness can be given by looking at the general wear and tear of the crystal faces (on an uncut 'rough' specimen) or flat surfaces (facets) and facet edges on a cut and pol­ished gemstone. A set of hardness pencils (each with a point made of one of the ten mineral specimens) can be used to test hardness and is particularly useful on carved pieces, crystal fragments and pebbles.

Google
Powered By Blogger

http://rpc.technorati.com/rpc/ping

http://rpc.technorati.com/rpc/ping