Natural red diamonds are rare enough that your best answer will come from a certified gemologist who can survey the diamond, document it and give you a valuation.
You can expect to pay for this certificate.
As an alternative, you can commission the manufacture of a man-made red diamond, below.
Using the query 'diameter' assumes that the diamond is a round brilliant cut. A 2.50 carat round brilliant cut measures about 9MM at the girdle -- your diamond's diameter will be slightly shorter.
A one-half carat diamond, in a round brilliant cut, measures about 5.2MM at the girdle.
A one carat diamond in the round brilliant shape measures aproximately 6.5mm at the girdle. If the diamond is another cut, measurements will be different.
A round, brilliant cut diamond measuring 6.5 mm at the girdle weighs about one carat.
A 1.00 carat round brilliant, when properly cut, should measure approximately 6.5 mm in diameter.
The value of a diamond depends on its cut, its clarity, its colour and its carat weight. A local jeweler can give you the answer you want.
Depending on the cut, a five carat diamond always weighs five carats. If the cut is round brilliant, the mm measurement at the girdle is about 11.
In terms of a round brilliant diamond, a properly cut stone should measure approximately 5.2 mm in diameter.
These two measurements are not directly related. If the diamond is cut in a round, brilliant cut, the carat weight may be between .15 and .20 carats. If the diamond is of a different cut, then the carat weight will be different.
It depends on a number of factors, namely the 4 C’s: cut, clarity, color and carat weight. However, the 1 carat round brilliant cut diamond D color, VVS1 clarity and excellent cut will cost you about $ 15,000.
A round brilliant cut gem of that weight measures in the 6.5mm range at the girdle.
A 20 carat diamond weighs 20 carats. If the diamond is a round brilliant cut, it measures about 17.6 mm at the girdle.