Part 4

This powder varies in color, and may be white, gray, red, etc. It is obtained by scratching the mineral with a sharp file, or by rubbing the mineral on the back of an unglazed porcelain plate, when the color of the powder will appear on the plate.

It is remarkable that the streak of the diamond is gray to grayish-black, while that of the ruby is colorless or white.

HARDNESS.

One of the most important and distinguishing qualities of a gem stone is the property of enduring, resisting wear,—in short, hardness. To test the hardness of precious stones that have not been cut or polished, the following scale of ten minerals has been devised by Moh, a German mineralogist:

No. 1. Talc. Very soft; is easily broken or scratched with the finger-nail.

No. 2. Rock-salt. Soft; scratched with difficulty with finger-nail; readily cut with a knife.

No. 3. Calcite. Low degree of hardness; not to be scratched with finger-nail; easily scratched with a knife.

No. 4. Fluor spar. Fairly hard; is slightly scratched by a knife, but easily attacked with a file.

No. 5. Apatite. Medium hardness; does not scratch glass, or only faintly; does not give out sparks against steel; easily attacked with a file.

No. 6. Felspar. Easily scratches glass; is attacked by a file, and gives some sparks against steel.

No. 7. Quartz. Quite hard; is only slightly attacked by file; gives sparks readily against steel.

No. 8. Topaz. Very hard; is not attacked by a file.

No. 9. Sapphire. Hardest of all minerals but the diamond; attacks all other minerals.

No. 10. Diamond. Attacks all minerals; is not attacked by any.

To find the hardness of a stone, begin to test with the softest mineral, so that when the number is reached which will scratch the stone, there has been no injury to the specimen under examination. Half numbers are determined by the ease or difficulty with which a stone is scratched. For example, a stone which will resist No. 7 (quartz) and which is only faintly attacked by No. 8 (topaz) may be safely put down as 7.5, while a stone which resisted No. 7 and yielded easily to No. 8 is to be classed as 7 in hardness.

These tests are readily applied to crystals or unpolished gems. With the polished stone greater care must be observed, and while a file test is often satisfactory, there is always the danger of striking the cleavage and breaking off a small piece of the stone.

SPECIFIC GRAVITY.

One of the most important tests which can be applied to a polished stone is that of specific gravity. Many stones, like the ruby and the spinel, the blue tourmaline and the sapphire, etc., look alike, but there is a sensible difference in their respective weights that a specific-gravity test will readily establish.

The weight of an object which is free to seek the centre of gravitation is called absolute weight, while the weight of an object compared with that of another containing the same volume of matter is called the specific weight.

If a stone weighing 16 carats is placed in a vessel filled to the brim with distilled water and the stone displaces 6 carats of water, the specific gravity of the stone would be 16 ÷ 6, or 2.66, the specific gravity of quartz.

In other words, the stone would weigh 16 carats in the air and only 10 carats in the distilled water, showing a loss of 6 carats, which is the weight of the volume of water equal in bulk to the stone;—or absolute weight, 16 carats; specific weight, 10 carats; loss, 6 carats; 16 ÷ 6 = 2.66, specific gravity.

There are several methods of ascertaining the specific gravity of a stone.

First, by placing it in liquids of known specific gravity.

Second, by weighing the stone in air and then in distilled water or alcohol, and thus learning the weight of an equal bulk of water.

Third, by measuring or weighing the water which the stone displaces when immersed in a small vessel of known capacity.

Fourth, by means of the Nicholson hydrometer, a simple instrument consisting of a hollow glass cylinder, two dishes, and a glass vessel.

As the jewelers’ balances are well adapted for the ordinary work of taking specific gravity, or can be easily adapted for such work, the second method will usually be the more practical to follow.

The author has had very satisfactory service from a $30 balance, and any well adjusted balance will give fair results.