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.

The following accessories are necessary to take the specific gravity of a stone:

Distilled water about 60° Fahr.

A very fine thread of platinum wire with which to suspend the stone (fig. 4).

A glass-beaker for the water (fig. 3, C).

A bench to hold the beaker over the pan (fig. 2).

The distilled water is easily obtainable from any druggist. The platinum wire should be bent to hook into the top of the balance frame, (fig. 3, B) and for ordinary small stones it will be convenient to twist the other end into a cork-screw shape or receptacle (fig. 4, A).

The beaker can be a small, thin glass cup of any kind, and the bench is easily produced from wood (fig. 2) or of metal with three supports (fig. 3, A).

To ascertain the specific gravity, attach the platinum wire to the balance frame, (fig. 3, B) and allow the lower end to rest in the water; then balance this carefully by adding weights to the other side (fig. 3, D) until the balance is exact.

The stone to be weighed in water is a ruby, and weighs two carats in the air.

Clean the stone carefully with water to free it from air bubbles; then place it in the screw of the wire (fig. 4, A) and weigh carefully. If the stone weighs 1½ carats it will have displaced ½ ct. of water: or, weight in air, 2 carats; weight in water, 1½ carats; loss, ½ carat; 2 ÷ ½ = 4, which will be the specific gravity of the ruby.

The Jolly spiral balance can also be used for taking specific gravity, but it is not so practical or accurate for small stones as for the larger ones.