A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public

by Frank Bertram Wade

A general understanding of the unit of weight in use for precious stones and how it is related to other standard weights is also necessary to the gem dealer. We will therefore consider in this lesson the use and care of balances and the nature and relative value of the unit of weight for precious stones.

Delicate Balances Needed. As it is necessary, on account of their great value, to weigh some gems, such as diamonds, emeralds, rubies, etc., with accuracy to at least the one hundredth part of a carat (which is roughly in the neighborhood of 1⁄15,000 of an ounce avoirdupois), balances of very delicate and accurate construction are a necessary part of the equipment of every gem merchant. While portable balances of a fair degree of accuracy are to be had, the best and surest balances are substantially constructed and housed in glass cases, much as are those of the analytic chemist, which must do even finer weighing. The case protects the balance from dust and dirt and prevents the action of air currents during the weighing. The balance itself has very delicate knife edges, sometimes of agate, sometimes of hardened steel, and these knife edges rest, when in use, on a block of agate or steel, so that there is a minimum amount of friction. When not in use the balance beam and knife edges are lifted from the block and held firmly by a metal arm, or else, as is the case with some balances, the post supporting the block is lowered, leaving the beam and knife edges out of contact with it. The object of this separation is to prevent any rough contact between the knife edges and the block on which they rest. Advantage should always be taken of this device whenever any fairly heavy load is put on or taken off of either pan, as the sudden tipping of the beam might chip the knife edges if not supported. When the load is nearly balanced there may be no harm in carefully adding or removing small weights while the knife edges are resting on the block, but even then it is safer to lower the beam and pans. It should be needless to state that as level and rigid a support should be had for one's balance as circumstances permit.

Method of Use of Balances. Before using a balance one should see that the pans are clean, that the base of the balance is properly leveled (the better balances have a spirit level attached) and that the pans balance each other without load. When slightly out of balance the defect may be adjusted by unscrewing the little adjusting nut at the end of the beam that is too light, or by screwing in the nut at the opposite end. Having seen that the adjustment is perfect the pans should be lowered and the object to be weighed placed on the left-hand pan (because a right-handed person will find it handier to handle his weights on the right-hand pan). One should next guess as nearly as possible the weight of the stone and place well back on the right-hand pan the weight that he thinks comes nearest to that of the stone. If the weight is too heavy the next lighter weight should replace it. Smaller weights should be added until a perfect balance is had, the small weights being neatly arranged in the order of their size, in order to more rapidly count them when the stone is balanced. This is the case when the pointer swings approximately equal distances to the right and to the left and there is then no need to wait for it to come to rest in the center.

It is well to count the weights as they lie on the pan (which is easily done if they have been arranged in descending order of size as suggested above) then write down the total, and on removing the weights count aloud as they are replaced in the box and note if the total checks that which was written down. It may seem unnecessary to be so careful in this matter, but it is better to be over-careful than to make a mistake where every hundredth of a carat may mean from one to five or six dollars or more. No dealer can afford to have a stone that he has sold prove to be lighter than he has stated it to be. One should be at least within one one-hundredth of a carat of the correct weight.

It should be unnecessary to add that accurate weights should never be handled with the fingers. Ivory tipped forceps are best for handling the weights. The forceps commonly used for handling diamonds will, in time, wear away the weights by scratching them so that they will weigh materially less. Unless the weights are of platinum or plated with gold, the perspiration of the hands would cause them to oxidize and gain in weight. It would be well to discard the smaller weights, which are most in use, every few years and obtain new and accurate ones. In case this is not done one should at least have the weights checked against others known to be of standard weight. Any chemist will have balances and weights far more accurate than the best in use for precious stones and will gladly check the weights of a gem dealer for a moderate fee.

To check the accuracy of your balance, change the stone and weights to opposite pans, in which case they should still balance.

One should never overload a balance, both because the balance might be injured and because the relative accuracy decreases as the load increases. If the weight of a parcel of stones heavier than the total of the weights provided with the balance is desired, the parcel should be divided and weighed in parts.

While many dealers neglect some of the precautions above suggested and somehow get along, yet it is safer to use care and to have correct technique in the handling of one's balances.

Having indicated a few of the refinements of method in weighing we will next consider the unit of weight in use for precious stones and see how it is related to other units of weight and in what manner it is subdivided.

The Unit of Weight for Precious Stones. The present unit for precious stones in the United States is the metric carat. Most of the more progressive countries have in recent years agreed upon the use of this unit. Its use in the United States became general July 1, 1913. It is by definition exactly one fifth of a gram (the unit of weight of the Metric System of weights and measures). Its relation to the grain is that there are 3.08+ grains in the metric carat. The carat in use in this country up to a few years ago was about 21⁄2% heavier than the present metric carat. It was equal to .2053 grams instead of .2000 grams (1⁄5 gram). The carats of countries not using the metric carat vary considerably, but yet approximate the metric carat somewhat nearly.