The gem's size is expressed in millimeters. If it's round, the diameter is measured. In other shapes, the length, width and depth are measured. This refers to the shimmery glow and iridescence on the pearl's surface. They are graded good, fair and poor.
Pearls can be smooth, slightly irregular, irregular, or very irregular.
Grading depends on the size, number, position and surface blemishes. This is the coating substance that makes a pearl what it is. The intruder substance implanted in the mollusk is coated by the host animal. This thickness can be determined by X-ray or observed with a jeweler's lens after the drill hole has been made. It can be graded thin, medium or thick.