Seylan Gems & Jewelry
People shop carat weight because it is a single number. The eye buys face-up size, color, and whether the center of the stone returns light. Those are not the same measurement.
Specifications
Typical published ranges used in gemology. Specific gravity below matches our carat calculator default where applicable. (SG 4.00)
| Carat | Weight. One carat is 0.2 gram |
|---|---|
| Millimetres | The size you see and the size a setting must hold |
| Window | A pale, see-through area in the face, often from a shallow pavilion |
| Extinction | Dark areas that do not return light, often from a deep or uneven pavilion |
| Spread | A stone cut wide and shallow to look larger for its weight |
| Calculator | An estimate from length, width, depth, and a species density — not a scale reading |
When the pavilion is too shallow, you can look through the table and see the finger or the background. That pale patch is a window. It makes color look washed out in the center, which is exactly where the eye rests. A small window in a light stone can be acceptable. A large window in a stone sold for its color is a reason to pay less, or to pass.
The opposite problem is extinction: the stone is so deep that parts of the face go black. You gain weight and lose life. Tilt the 360 view, if there is one, and watch whether the face flashes or goes dark.
Write down carat weight and the three millimetre measurements. Two sapphires at 2 carats can differ by millimetres of face-up width if one is deeper. For a ring, the width across the finger is what people notice. For a pair of earrings, matching millimetres matters more than matching carat to the second decimal.
Our carat calculator multiplies length, width, and depth by a specific gravity and a shape factor. It is a sanity check. A scale is the weight. If the estimate and the listed weight are far apart, the depth, the shape factor, or the species density may be the reason. Ask before you decide the listing is wrong.
A higher price per carat on a smaller, finer stone can be the better buy. A lower price per carat on a windowed or heavily treated stone can be the worse one. Compare species, color, treatment, and the face you actually see. Then compare price.
We would rather you reject a stone with a dead center than buy it because the carat number was large.
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