Weights and values of gemstones
Nearly all gem stones are sold by weight, and since they are relatively small in bulk, a correspondingly small unit of weight is most convenient to use in arriving at their weight and therefore their Value. The almost universally recognized unit now in use is the carat, although there are instances when the troy ounce or the metric gram are used. These larger units are more convenient when rough stones, or larger specimens of the less valuable varieties, are being considered. Incidentally, the carat weight has no connection with the gold carat standard, which merely indicates how many parts in 24 (or 1000) are gold, the rest being alloy.
Precious stones are calculated as being worth so many dollars or pounds a carat, according to the currency of the country in which they are being offered for sale. Their value will also depend upon other factors, such as prevailing market prices, individual qualities, the presence of faults, color, and perfection of cutting.
Since the carat is a small unit of weight, it follows that a slight difference in weight will make a great difference in value, especially when stones are rated at so many dollars or pounds per carat. For this reason, exact weighing is necessary, and a delicate balance accurate to one-hundredth of a carat or a gram should be used.
Weight has been considered long before the gem has assumed its final form, for the lapidary has had this in mind from the time he has handled the rough crystal. He has had to decide on how the stone should be cleaved and cut, retaining weight but cutting out bad flaws, at the same time producing a well proportioned gem. Small imperfections are generally left if their exclusion would mean a waste of much material.
We might say that the value of a gem stone depends upon demand, rarity, weight, and nature of internal flaws, the amount of deviation from the recognized ideal shade, and perfection in shape and cutting. But values do not increase in simple proportion to size. A three carat stone is worth much more than three times the value of a one carat stone of the same quality.