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pearl grain

1

In the United Kingdom, a unit of mass used in the pearl trade, such that 600 pearl grains = 1 troy ounce, so 1 pearl grain = ⁴⁄₅ of a grain, about 51.839 milligrams.

sources

Pearls are either sold by the piece according to their weight, or by the ounce troy, which is divided into 600 grains; this is the custom with all dealers in pearls. Single pearls of value are sometimes sold by diamond grains.

Report of the Commissioners Appointed to Consider the Steps to be Taken for Restoration of the Standards of Weight and Measure.
London: Printed by W. Clowes and Sons for Her Majesty's Stationery Office, 1841.
Page 57.
Testimony of Granville Sharp, “of the firm of J. G. and G. A. Sharp, Bullion-brokers.”

2

In Australia and New Zealand, the grain, as applied to pearls, was ¼ of a carat of 205.5 milligrams, or 51.4 milligrams. This unit, perhaps coincidentally, bears the same relationship to the carat (¼) as the old Islamic pearl grain, the ḥabba, bore to the ḳirāṭ.