The hydrate of sodium sulphate is known as Glauber's Salt after the Dutch/German chemist and apothecary Johann Rudolf Glauber (1604–1670)
The underside is clearly stamped with the orb and cross trademark Benham and Froud used from 1855 (when they took over the company from Richard and Edward Kepp)
or failing that
Kazaks often display two or three geometrical medallions
Pringle and Booth of Toronto and many are from “the collection of Hunt Bushnell”
A Beautiful Post Medieval Silver Bifacial Cross Pendant; ex Sir Francis Cook Collection sub-Antiques The hydrate of sodium sulphateOrigin: English Period: Mid Nineteenth Century Provenance: Silk collection, London, UK; ex Czechoslovakia auction, Kolekce Malych Krizu PI. Refordove, a collection of 87 cross pendants from the collection of Mrs R. W. Reford, the collection originating in the course of the 1920's in England; ex francis Mallet, Malllet & Son, New Bond Street, London; ex Sir Francis Cook (1817 1901) collection, 1st Baronet, British merchant and art collector. Date: 1840