An old mine cut diamond that measures 7mm possesses one measurement that is 7mm.
No other data about the diamond can be deduced about the diamond given this measurement.
Since cushion cut is a rather square cut, one must assume that your diamond measures 7mm x 7mm. In this case, it weighs approximately two carats.
If a diamond is a round cut, the mm size at the girdle is about seven if the diamond weighs about 1.25 carats. Other gemstones have different weights relative to their mm measurements.
A round cut diamond measuring 7 millimeters across is roughly the equivalent of 1.25 carats. Millimeters is a unit of measurement that describes the size of an object, and carats are used to measure the weight.
A brilliant cut diamond of one carat weighs exactly the same as a mine cut of one carat.
Grammatically, mine cut is proper. You can read more about how this cut got its name, below.
You may be thinking of a mine cut, a cut of diamond favoured in the early 1800s. You can read more about this cut and its place within the history of diamond cuts, below.
'Perfect' is relative. A diamond is valued by its cut -- mine cut in this case, its carat weight, its clarity and its colour. You can take your stone to a local gemologist who can help you establish a value for this diamond.
This millimeter measurement across the girdle of a round cut diamond may be equal to about 1.25 carats in weight. A certified gemologist can give you the exact weight of your diamond.
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
Size -- or weight, is one determinant. Others include the diamond's colour, clarity and cut and its excellence.
The size of the stone used in a 2ct princess cut diamond engagement ring is 8.2mm. This size is measured using a pair of brass calipers. The size can vary, depending on how deep the diamond is, but as a guide 8.2mm is a fair estimate.
7MM metric hex. Sears has them in the socket hex for $11. Get the long socket hex and cut it to size.