The value of a given pearl depends on the combination of a number of different factors. And depending on the type of pearl, one value factor might be more important than another.

Luster

Luster is the combination of a pearl's surface shine (reflectivity) and inner light refraction (depth), and is perhaps the most important value factor. Pearls with high luster are shiny and bright and exhibit an iridescence known as "orient". Pearls with low luster tend to look chalky and dull. Luster is closely related to nacre quality.

Nacre Quality

When a mollusk senses an irritant within its body it secretes layers of semi-opaque crystalline substance composed of calcium carbonate that coats the irritant. This substance, called "nacre" (nay-ker) builds up over time and becomes a pearl. The thicker the nacre, generally the more lustrous the pearl. Although there are exceptions to that rule, the amount of nacre determines the reflective quality of the pearl's surface.

Greater thickness of the nacre also increases the durability of the pearl. Interestingly, freshwater pearls, because they are nucleated with tissue, form pearls composed entirely of nacre and can therefore exhibit very high luster and are quite durable.

Surface quality

A quality evaluation category used to describe the number and severity of blemishes on the surface of a pearl. Surface descriptions range from "clean" (no visible blemishes) to heavily blemished.

Size

Larger size pearls of good quality are more difficult to grow than smaller sizes and therefore rarer and more costly. 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 surface quality of the pearl. The greater surface area makes it more likely that the pearl will develop blemishes.

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.

Color

Pearl colors vary greatly and beauty is largely in the eye of the beholder. The market for classic pearls generally favors silvery white or white with pink overtones (white rose), with creamy colors less sought after. Tahitian pearls are most highly prized when approaching black with pink and green overtones, known as "peacocks." Freshwater pearls exhibit a very pleasing range of colors including peach and lavender, sometimes referred to as "spice pearls."

Shape

It is rare for a pearl to be perfectly round, even if the nucleus around which it grew was perfectly round. Therefore, rounder pearls are scarcer and more valuable. "Baroque" pearls which are extremely asymmetrical, can be quite interesting and striking, and are significantly less expensive than round pearls of similar size and quality. Slightly off-round pearls of good quality can represent good value as they look round to the casual observer, and cost less than round pearls.

Practical advice:

The prettiest pearls are those with the best luster. Slightly off-round pearls with slight blemishes and high luster look virtually the same as top quality pearls to the casual observer and are significantly less costly. To stay in a budget make small compromises in roundness, surface quality and size, but stay with good luster. Pearls with thin or poor quality nacre should be avoided, even if they are very round and have clean surfaces. If the nacre is thin or of poor quality, the pearl will usually look dull. It may also be possible to see the bead nucleus beneath the nacre surface.

Freshwater pearls can be amazingly good values. Not only are there many shapes and colors to choose from, but very roundish freshwater pearls are being produced in larger numbers, better qualities and bigger sizes today than ever before. Because freshwater pearls are tissue nucleated, they are "all-nacre" pearls and as such can be very lustrous and durable. Freshwater pearls are generally much less expensive than comparable saltwater pearls.