Carat (ct) is a measurement of weight, not size. There are five carats to a gram. A 1 ct sapphire is smaller in size than a 1 ct diamond because it is heavier. There are 100 points to a carat, so a 50 point diamond weighs 1/2 a carat or 0.50 ct
Diamond prices are usually quoted per carat. Therefore a 0.50 ct stone quoted at $4,000 per carat would cost $2000. As the carat weight increases for the same quality of diamond, the price per carat goes up. So a 1.00 ct stone of the same quality might cost $8,000 per carat.
Also the price per carat will jump as you approach the next higher carat weight. Therefore a 0.95 ct diamond of the same quality as the 1.00 ct stone above might cost $7,000 per carat or $6,650, but look almost as large.
The term spread refers to the size a diamond appears to be, based on its diameter. So a 0.90 ct stone with less depth might look the same size as a perfectly cut 1.00 CT diamond, but has less brilliance because it is cut shallow.
This is why it is important to understand the basics of color, clarity, cut and carat when reading a diamond grading report (sometimes called a certificate). All four factors influence both the beauty and the price of a diamond. Evaluating the value of a diamond by color and clarity alone can lead to a costly mistake because the quality of the cut is a very major factor governing a diamond’s appearance.
A proper diamond report should give the following information:
1. The date issued, 2. Who issued it, 3. Identify it as a real diamond, 4. The exact carat weight, 5. The dimensions including narrowest and widest diameter and depth, 6. Proportioning–table and depth percentage, 7. Color and clarity, 8. Polish and symmetry, 9. Girdle thickness and crown angle.
The final test is the overall appearance. Every diamond will have its faults, and even with major faults a stone can be very beautiful. If it is a good looking diamond and fits your needs, buy it.