Cut refers to proportions and symmetry of a gemstone, and it should highlight its color and brilliance, while diminishing inclusions.
The anatomy of a gemstone
Like diamonds, color gems usually have a table (the top surface), girdle (the middle, widest part of a stone), crown (the top portion between girdle and table), pavilion (the cone-shaped lower part), and culet (the facet at the bottom tip, preferably not visible with the unaided eye).
Certain gemstones, including opal, turquoise, onyx, moonstone and star sapphire, are not faceted. Instead, they are polished to form a smooth, rounded or outward-curved shape called the cabochon cut , usually with oval outline.
Tips for buyers
There are no uniform cutting standards when it comes to maximizing brilliance or color. A well-cut gemstone is symmetrical (so that its facets reflect light evenly across the surface), with a smooth polish that leaves no nicks or scratches.
A gemstone with highly saturated color might benefit from a shallow cut, which will allow more light to pass through. A deeper cut may enhance the color of a less saturated stone. Rare gemstones are sometimes cut to emphasize their size and weight, rather than color and beauty.