Colored gemstones are usually measured by carat weight. It’s important to note that density can vary between gemstone types. A sapphire, for example, is denser than an emerald. So, a sapphire will actually be smaller than an emerald of comparable cut and weight. In the end, the perceived size of any colored gemstone will depend not only on its weight, but also on factors such as its shape, style, and setting. For this reason, carat weight is not often described in a product listing..
Gemstones are measured in millimeters (mm). A round gemstone is referred to by its diameter. Popular sizes for stud post earrings would be 4mm, 5mm and 6mm. Dimensions of other shapes are expressed by length, width and sometimes depth. A 7x5mm rectangular stone would be 7mm in length and 5mm in width.
A cabachon is a gemstone that is not faceted. It is cut into a convex shape - often rounded and then polished. The shape may be symmetrical, calibrated or freeform. Opals, turquoise and softer stones are cabachon cut.
Today, most gems are treated to improve appearance or durability. The heat treatment of gems is the most common treatment which has been used for thousands of years. In ruby and sapphire, the treatment is often performed to improve color and clarity.
Gemstones such as turquoise and opal would be too fragile without stabilization and impregnation treatments to improve their durability. The traditional treatment for emerald is fracture-filling with natural oils such as Cedarwood oil in order to improve their clarity. Almost all colored topaz is irradiated to bring out its color.
Cut describes the silhouette of a colored gemstone — a combination of shape and, often, style. Different stone types may favor particular cut and different cuts absorb and refract light differently which may affect their sparkle. Beautiful stones can be found in virtually any shape or style.