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.
The valuable precious stones are bought and sold by the carat. This weight is equal to about 3.17 grains or about .205 milligrams.
The carat is divided into fractions of ½, ¼, ⅛, 1/16, 1/32, 1/64, and also arbitrarily into four grains; that is, each quarter of a carat is counted one grain, thus forming the basis for the calculation of pearls.
In commerce, a carat diamond is sometimes called a four-grain stone, and a carat-and-a-half stone is six grains, etc., etc.
The weight of the carat being arbitrary, it varies in different countries, some being heavier and others lighter than .205 milligrams.
The writer wrote to three prominent balance-makers in the United States some months ago for their carat standards and was surprised to find that they all differed. This will account for discrepancies in weight resulting between the balances of different makers. Of late there has been a decided movement in Europe, headed by the French Chambre Syndicale of jewelers, in favor of the unification of the carat, so that the weight of a French or Dutch carat will equal that of an English, American, or any other carat. This reform will probably be accompanied by the adoption of the decimal system of dividing the carat, and the discarding of the complicated fractional system.