Pearl size refers to the pearl's diameter, which is measured in millimeters. Normally, the larger the pearl, the more rare it is and the more valuable. Keep in mind that pearl strands also have a length, which also affects price because of the number of pearls needed to create that particular necklace length.

The size of the oyster directly affects the size of the pearl it creates. Larger mollusks, for instance, create larger pearls.

However, large cultured pearls require a larger nucleus to be implanted into the oyster -- which increases the chance of the mollusk either rejecting the nucleus, or dying before a pearl has an opportunity to develop.

A large nucleus also affects the complexion of the pearl. It's more likely that a larger, mature pearl will be formed with greater surface defects.

Round pearls are measured according to their diameter, while baroque pearls are measured by their length and width. Size is determined by diameter and expressed in millimeters, since pearls are sold by the millimeter.

Pearls, of course, come in a range of sizes, these being the average:

• Akoya: 6 to 8 millimeters in diameter, with 8 mm considered large.
• Freshwater: 5 to 6.5 millimeters, for the best quality.
• South Sea: 11 and 13 millimeters, large by any pearl standard.
• Tahitian: The match of the South Sea pearl, also between 11 and 12 millimeters.

Pearl strands are sorted by size, with a specific millimeter range:

• Petite: 6 millimeters and below.
• Classic: 6 to 7.5 millimeters.
• Most Popular: 7 to 7.5 millimeters.
• Outstanding: 8 millimeters.

A pearl's size doesn't necessarily indicate its quality, but it most certainly does affect its price. All things considered, the larger the pearl, the higher its value.