Sizes of pearl
Round and semiround pearls are usually sorted by size, drilled, strung onto silk thread and sold as 16-inch (40-cm) rows. They can also be sold as pairs and singles.

FRESHWATER PEARLS start at about 2.5 mm and increase in 0.5-mm increments to 9 or 10 mm. The largest freshwater pearls are 10-12 mm. This rare freshwater pearl measures 16 mm in diameter.

AKOYA CULTURED PEARLS are also graded by size: a diameter of 2-5 mm is small; 6-7 mm is average; 8 mm is large; 9-11 mm is very large and the maximum size possible from akoya oysters. The prices jump up with each increase of size.

SOUTH SEA AND TAHITIAN PEARLS in rows start at 8-9 mm, with a maximum size of about 14 mm. The price of a row of Tahitian pearls is high, reflecting the difficulty of finding and matching enough pearls of the same shape, size and colour. Black South Sea baroque pearls measure between 9 mm and 20 mm,
MABE PEARLS are produced in sizes ranging from 10 mm to 17 mm in diameter. These mabe pearls measure 17 mm in diameter.

These come from the akoya oyster and are typically round in shape. The natural body colours range from light pink to white to pale yellow, and include green and blue-grey tones. Akoyas are normally associated with Japan, especially if they measure over 7 mm. China, Korea, Hong Kong and Sri Lanka produce akoya
pearls in smaller sizes.

These pearls are cultured in the large mussel Pinctada maxima and usually require a longer growing time in the shell than do akoya pearls. They can be white with a rose or green tint, green, blue-grey, golden or pale yellow in colour. The lustre on light-coloured South Sea pearls tends to be less intense than on the dark pearls. Vibrant golden – coloured South Sea pearls are very popular and command a high price, as does the white-rose colour. South Sea pearls are the largest cultured pearls of all and as their size increases so does the price. Round pearls are the rarest and most expensive shape.

Tahitian pearls from the black-lip mussel Pinctada margaritifera are the only natural black pearls that exist; all other black pearls are dyed. The “black”
colour can vary from silver to dark grey and may have pink and green overtones. Black pearls can look almost metallic.

Keshi are tiny pearls that form spontaneously when a much larger nucleated pearl is cultured in the akoya oyster. As keshi grow without a nucleus, they are, in effect, “natural” pearls. They have the same colouring as akoya pearls. South Sea and Tahitian pearl molluscs also produce keshi, but in a much larger size. These can exceed 10 mm in length and are often used in jewelry because of their interesting shapes.

Shapes of pearl
Pearls come in a large variety of different shapes, making them fascinating to work with. The shape of a pearl depends on the type of mollusc it grew in, and on how the mollusc reacted to the nucleus.

If the irritant in the mollusc stays unattached to the shell, a nearly spherical or off-round pearl can develop. Perfectly round pearls are very rare.

When an irritant, or nucleus, becomes attached to the shell, irregular shapes can develop. South Sea and Tahitian pearls grow into some amazing baroque shapes because they have thicker nacre layers and spend a longer time growing in the mollusc. The irregular surface generally emphasizes the pearl’s lustre and iridescence. Keshi pearls also have baroque shapes and they can be bought singly or as a row.

Drop pearls are nearly as difficult to grow as round pearls. As a rule, the more symmetrical the drop shape, the more expensive the pearl. Drop pearls need evenness, symmetry, good proportions and balance. Smaller drop- shaped pearls are relatively easy to find, but sizes from 8 mm are rarer.

Multiple and twin pearls occur when two or more pearls accidentally join or fuse together; sometimes a specially shaped nucleus is used. Bicoloured pearls are also described by this term. Fancy pearls are great for one-off designed pieces they are relatively rare.

These Chinese freshwater pearls are grown at great speed. The wrinkled irregular appearance of the rice pearls is a direct result of their very short cultivation period. Potato-shaped pearls can be large in size, but their lustre tends to be poor. They are often circled (with rings around them) and can be pitted.

These are grown in a similar way to mabe pearls. They are generally used for earrings and are supplied with a drill
hole to accommodate a metal post for fixing. Button-shaped pearls also come as a row.

Three-quarter pearls and half-pearls are produced specifically to be mounted on ready-made stud earrings or rings. When they are set onto the metal fittings the pearls often appear to be full-round in shape.

Chinese freshwater pearls resemble Japanese Biwa pearls, but they use a nucleus. Instead of a round mother-of-pearl bead, mantle tissue from another oyster is cut into the desired shape and implanted. The colours are wide-ranging, but include rose, white, green- white, green-rose, salmon-orange, wine red and violet.

Lake Biwa is Japan’s largest lake and was the first freshwater culturing site. Biwa pearls are noted for their good quality, smooth surface and high, even lustre. They don’t normally have a nucleus as the mussel won’t accept one, and as a result some bizarre shapes can occur. The colours range from creamy white to white-rose, salmon-orange, dark wine red and violet. Many freshwater pearls on the market are called Biwa pearls, despite being cultivated in China. This is probably done to impress or reassure customers, and to increase price.

Mabe pearls are cultured by gluing a half- bead nucleus against the inside of the shell. When the hemisphere pearl is covered in nacre it is cut out, the nucleus is removed and the hole is then filled and the pearl backed with mother-of-pearl. Because mabe pearls are constructed they are not as durable as other types of pearl, and over time the nacre coating can either lift off, become damaged or sometimes discolour. If a mabe pearl has a rim around it, making it look like a fried egg, it is called a blister mabe.

These pearls grow attached to the inner surface of the shell rather than loose in the mantle. They have the same iridescent nacre as the inner surface of the shell and the back is flat, without any pearly coating.

Seed pearls are small natural pearls that measure 2 mm or less. Because of their size, they are usually drilled and strung in a country where labour is cheap. People often choose not to restring seed pearls as they are so small and fiddly to work with.