Page:Handbook of Precious Stones.djvu/29



As the property of hardness is of great value in the case of precious stones, those kinds which are scratched by quartz, and which, consequently, are below 7 degrees of hardness, are ranked as half- hard, or "demi-dures." Stones scratched by a knife are below 5 degrees.

There are two remarks as to degrees of hardness which it is proper to introduce in this place. Firstly, the degree of hardness of a crystal or a cut stone varies, generally, however, within narrow limits, on different faces and in different directions. Secondly, the usually accepted scale of hardness is one having very different values for the different intervals. Unlike the degrees of the thermometer where the interval between one degree and the next above it or below it has the same value, whatever part of the scale be chosen for comparison, the degrees of hardness on Mohs' scale show extraordinary divergences.

Specific Gravity.—The most generally applicable of all modes of discriminating precious stones from one another is to ascertain their specific gravity—that is, the relative weights of equal bulks—the weight of a bulk or volume of distilled water (commonly taken at 60° F. or 15.6° C.) being employed as the unit with which all the others are compared. There are three modes of ascertaining the specific gravity of a stone: (1) By placing it in heavy liquids of known specific gravity, and noting the position which it takes up. (2) By weighing it in air, and then in water (or other liquid), and