You are doing an appraisal and have a mounted tennis bracelet that weighs a total of 15 pennyweights on your gold scale. How would you deduct the weight of the stones to arrive at the actual gold content? On a larger scale you're weighing rough stones and wish to purchase one kilo (kilogram); do you know the pound weight required for a kilo? Do you know if you are using an avoirdupois or a troy scale?
Questions like these arise daily, since we use such a wide variety of scales in the jewelry industry. Gold is weighed on a pennyweight or gram scale. Gems are weighed on a carat scale. Postage is weighed on an avoirdupois scale and pearls are sometimes weighed on a momme scale. Do you ever need to convert the results of one scale to those of another, because the scale you needed was not handy at the moment? We set about compiling a new chart to help jewelers interpret and convert outcomes from various jewelry scales.
We began with the usual charts, which are found in most gem books and manuals, in an effort to find a common denominator to connect all the systems we use. We found that troy, avoirdupois and apothecary weights all start with a unit called a "grain" which is equal to 0.0648 grams. Although grains are not in common use with metal weights anymore, historically we have used this unit of measure. Did you know that 24 grains equals one pennyweight? We thought we were on the right track until we discovered that the number of grains in the troy ounce is not the same as the number of grains in the avoirdupois ounce. To make matters worse, we learned that this grain is not the same as our "diamond grain" unit. When we use the term "4-grainer," we mean one carat, and our carat is equal to 0.2 grams (not 0.0648 grams). Bottom line: Be sure that if you have an avoirdupois scale, you do not use these grains for gem weight without using the chart below to convert the figure.
The ounce itself also poses a problem. The troy pound and the avoirdupois pound have different ounce measurements; there are 12 ounces to the troy pound and 16 ounces to the avoirdupois pound.
It's important to understand that the gram scale and the carat scale are based on the metric and troy systems, and are mostly used for smaller items. A common unit is the gram and both our metal weight and our gem weight can be converted to one another if necessary. The avoirdupois scale (the kind we use in the mail room and in grocery stores) evolved separately and is mainly used for larger items. We still use it for weighing larger volumes of rough stones when we use the term "kilo." If you check the chart, you will see that a kilogram equals 2.2 pounds on the avoirdupois scale.
We intentionally put the "grain" unit at the end of the chart so that it would not be confused with the diamond term. (In fact, this unit is sometimes also used with pearls.) To be clear, a diamond or pearl grain weighs 0.05 grams. A troy or avoirdupois grain weighs 0.0648 grams. It can be easy to confuse these! Also an old unit of weight for pearls is the "momme." We've included the momme in the chart so that weight can be measured with other scales and converted to mommes.
If you use a good postage scale (avoirdupois weight) to weigh gold or stones, and you convert that avoirdupois weight to grams and that gram weight to pennyweights or carats, you will only get an approximate weight. It is important to remember that this scale does not have the narrow tolerances of the scales normally used for gems and metals and is NOT the best choice for small items.
You will find the chart extremely helpful if you have to weigh a mounted piece such as the one mentioned above. Once you have the carat weight of the stones, the total weight of the piece can be converted to grams or pennyweights and you will be able figure out the gold content of the piece by subtracting the stone weight.
For example: the complete tennis bracelet has 25 stones, with a total weight of 2.5 carats, and weighs 15 pennyweights. Using the chart you can see that 1 carat weighs 0.1286 pennyweights. Multiplying 2.5 by 0.1286 we get the total weight of the stones as 0.3212 pennyweights. Now subtract 0.3215 from 15 and we get 14.6715 pennyweights of metal in the piece.
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