Usually the pearls when round and lustrous are not constructed as compactly as those of the bivalves. The texture of the skins vary in quality and the frequent presence of intermediary strata of black conchiolin which shrink, makes them liable to crack and break. The blisters run very even in these two qualities of color and luster and though seldom quite as brilliant as the nacre of the shell, are very beautiful and often curiously formed. These blister-baroques are like two blisters joined at the edges, and are liable to separate there. The interior consists chiefly of black conchiolin, rough and somewhat shiny. The "Conch" pearl, found in the Conch (Strombus gigas) of the West Indies, also is not a true pearl. The shell is used largely for ornamental purposes, especially for the cutting of cameos, and also in porcelain works. It is a large shell, sometimes weighing four or five pounds. Formerly great quantities were exported to England from the Bahamas; in one year as many as three hundred thousand. Conch pearls are devoid of nacreous luster, the surface having an appearance like china. They are slightly transparent and show under the surface a series of delicate wavy markings. The silky sheen of these lines causes them to appear lighter than the body color of the pearl, and they seem to branch toward the surface, changing kaleidoscopically as the pearl is turned. Almost without exception the shape is ovoid, or a flattened ovoid, though some are distorted. In color they range from very pale to deep pink and coral red, the ends being usually much lighter than the body and often white. In the deeper tints they are more uniform in color, and as they are apt to be less lustrous and transparent as the shade deepens to red they show less plainly the distinguishing wavy lines, and may be easily mistaken for pieces of coral cut to the shape and polished. They are very delicate and therefore easily fractured or cracked. As the natives usually obtain the pearls by cooking the fish, for which they have a great liking, a large proportion of the few which come into the market are cracked. It is claimed also that the color fades with time. They are sometimes called "Nassau" pearls. Pearls similar in appearance to the Conch, except that the wavy lines are absent and the skin rarely as brilliant, are taken with true pearls from the small varieties of the Avicula, especially about the coast of Venezuela. Some are white as chalk, many are tinted in various shades of gray, yellow and brownish reds. They have the shining appearance of china in different degrees, but no nacreous luster. The skins of many of these are peculiarly constructed, they show modified characteristics of various parts of the shell. The surface wave lines are present to some extent, together with curious malformations of prisms and conchiolin.