Physical Characteristics of Precious Stones: Weight and Molecular Action

WEIGHT AND MOLECULAR ACTION:

Specific gravity.--Every one knows that two equal volumes of different substances have seldom the same weight: a piece of lead, for instance, is much heavier than a piece of wood exactly equal to it in size. If we find the weight of a substance, and also that of an equal volume of another substance, selected as a term of comparison (distilled water is the term that has been chosen), and if we divide the weight of the first body by that of the second, we obtain a number which expresses how many times, and fractions of times, the body considered is more or less heavy than that to which it is compared. The number thus obtained is its specific gravity.

In the case of precious stones it is a characteristic of extreme importance, for it is frequently the means by which the difference is detected between stones that the eye might easily confound. In this way, for example, the diamond can be at once distinguished from the zircon, the specific gravity of the former being 3.4, and that of the latter 4.4.

Hardness.--We should be careful not to fall into the very frequent error of confounding the quality of hardness with that of resistance to crushing or concussion. There are minerals that may be crumbled between the fingers, and that are yet none the less hard. The hardness of a substance, according to the definition of Delafosse, is "the resistance which it opposes to the action of a point like that of a steel needle which may be drawn across it, or to the angular part of another mineral passed with friction over it surface."

Hardness is an indispensable quality of gems. If a stone were not very hard, the continual friction to which it is subjected would very soon destroy its polish; and with the polish, transparency, brilliancy, fire--all that constitutes its value--would vanish.

It is owing to this quality of hardness, added to the unchangeable nature of their substance, that stones, cut perfectly by Egyptian artists thousands of years ago, have reached us intact; and give us the most interesting proof of the progress in arts and civilization which had been attained in those remote periods.

Fusibility.--Fusibility is the property which solid bodies possess of passing into the liquid state, when they are subjected to a sufficient temperature.

For precious stones in particular the point of fusion is lower in proportion as the composition of the stone is more complex. Thus the diamond, a simple body, is absolutely infusible. The ruby, the sapphire, the topaz, binary bodies, can only be melted before the oxyhydrogen blowpipe. The simple silicates, ternary bodies, are fusible at a much lower temperature; and the multiple silicates offer no serious resistance.