This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Perlae. Uniones. Pearls: small calculous concretions, of a bright semitransparent whiteness, found on the inside of the shell of the concha margaritisera or mother-of-pearl fish, as also of certain oysters and other shell-fishes. The finest pearls are brought from the East and West Indies: the oriental, which are most esteemed, have a more shining silver-like hue than the occidental, which last are generally somewhat milky: an inferiour sort is sometimes met with in the shell-fishes of our own seas, particularly on the coasts of Scotland, The coarse rough pearls, and the very small ones which are unfit for ornamental uses, called rag pearl and seed pearl, are those generally employed in medicine.
It is said, that counterfeit pearls are often brought from China, made of pellets of clay coated with the white matter of oyster-shells. The clay may be distinguished by its acquiring an additional hardness in the fire, and resisting acids; whereas the true pearls calcine in the fire, and become quicklime, and readily dis-solve in acids; the vitriolic excepted, which precipitates them when previously dissolved by other acids.
These properties of the pearl, shew that it is an earth of the same kind with crabs-claws, oyster-shells, and the other calcareous animal absorbents. It has no other virtues than those of the other substances of this clafs, and does not poffefs those virtues in any greater degree than the common testacea,