Many faults can be concealed by the jeweller when the pearl is mounted. Slightly buttoned pearls are set on a peg in the centre of a small shallow cup; they then appear quite round. A spot, blister, or cavity, in a round pearl can be obliterated by pegging, or hidden in the setting. Great irregularities in the sphericity are lost to the eye when the gem is set in the prongs of a ring or other piece of jewelry. Pearls shaped like a double convex lens may be made to look round, or very nearly so, by piercing them so that the flattened domes are brought in contact on the cord holding them together as a necklace. Piercing and stringing obliterates or hides many flaws. By careful selection, the jeweller can utilize pearls having a blemish by drilling through the spot where the flaw is, and if there is another on the opposite side that also will disappear. Other imperfections near the hole are often hidden in necklaces, as they cannot be seen when the pearls are held close together on the string. It is for this reason that a string of pearls can often be bought for less than a like number of loose pearls apparently no better but which in reality are much more perfect in shape and free from flaws. Imperfections unseen in the strung pearls would be quite noticeable in the loose and undrilled. The irregularities of baroques cannot properly be called imperfections; nevertheless a baroque is more valuable as it is free from indentations and approaches the round in appearance, or has sides which will give it a round face when mounted. The curious forms into which nature moulds many of them are very attractive, and as they lend themselves to the imaginative skill of the jeweller, are valuable. The faults common to them are rough places uncovered by nacre and colored streaks or spots, usually yellow tending to brown. These discolorations are confined generally to the point where the baroque was attached to the shell, but not infrequently they extend far enough to leave no front which would be quite clean to the eye, when mounted. Oriental baroques as a rule are more lustrous, more even in shape and seldom discolored. Many of them are sufficiently regular to string for necklaces, and some can be used in jewelry so that on the face they appear like round, drop, or pear-shaped pearls. GENESIS OF PEARLS [Illustration: TUAMOTU PEARL-SHELL] Pearls are found in certain marine and fresh-water mollusks. The former are usually termed oysters, though zoölogists regard it in some instances as a misnomer. The sea-fish is the avicula margaritifera, a bivalve of which there are many varieties, all of similar shape and nature but differing widely in the size, weight, coloring, and quality of the shell.