The weight of a diamond is determined by means of a diamond balance. The balance should stand on a table or shelf which is free from shocks and vibrations, as the smallest vibration can influence the accuracy of the measuring. The balance should also be placed out of the way of drafts or heated air streams, as variations in the temperature-can also influence the accuracy.
Although it is commercially. acceptable to give the weight of a diamond up to the second decimal place, many reports give the weight in thousandth of a carat, i.e. up to the third decimal place.
To round up the second decimal place is only allowable if the third place is a nine:
0.998 = 0.99 ct
0.999 = 1.00 ct
It is nearly impossible to weigh accurately to the third decimal place on a mechanical analytical balance, the results for the same stone could vary up to four thousandth of a carat.
In order to obtain maximum accuracy with a mechanical balance the zero point must be adjusted every time before the balance is used.
The stone is carefully placed into the pan with tongs; care should be taken not to vibrate the pan too much. The glass sliding doors situated at the side of the balance must be closed during weighing in order to exclude any draft. When the balances are not in use, this glass should also be closed to exclude any dust.
The balances are then switched on and the result can be read off on a lit graduated scale as soon as the pan is at rest. Then the balances are switched off again and the stone carefully removed.
Operation of the electronic carat balance is simplicity itself. On-and-off switching, zero settings and tare can be carried out at the touch of key. The weighing pan with one or more stones is placed in the balance and the figures of the green-light display show measurements quite clearly, even under conditions of bright lighting and from an unfavorable viewing angle. The most commonly used jeweler's balance the CM 200 (Fig 389) has a weighing range of o to 200 carats and can be converted to a gram reading at the touch of a key. The carat read-out can be read off continuously up to 1/1000 ct, and up to 1/100 gram continuously in the case of the gram reading, while the results can be reproduced at any time. The reliability of the balance can be tested as required by use of the test stones delivered with the balance for checking its accuracy.
The at-rest position of the zero point is controlled and there is a practical arrangement which enables one to adjust the measuring cycle so that reliable results are also obtained if the place of weighing is not quite free of vibrations. When the pan is at rest, a "zero" appears when a certain key is pressed: when the pan is vibrating, this "zero" disappears shortly and is only lit up again when the zero point has again been reliably reached.
After the stone has been placed onto the pan, a light signal informs the user of the reliability of the last decimal point. In a few seconds the precise weight can be read off.
The electronic carat balance can also be used for the weighing of specific gravity in the determination of diamond imitations or in the examination of colored stones (Fig 390). Additional accessories can be bought for these purposes. The normal pan is removed and a special pan with stone holder substituted: A glass bowl with distilled water is put on a metal bridge which is placed over the special pan. The stone if first weighed in the upper stone holder in air (dry weight). Then it is placed into the lower metal basket, one reads off the weight in water and calculates the density of the stone from the loss of weight :
Weighing of specific gravity = weight in the air / loss of weight
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| Fig 389 Electronic carat balance (made by Mettler) | Fig 390 Electronic carat balance for measuring specific gravity (model Mettler) |
| Unit of Weight | Weight Tables |