Topaz Enlarge
June 1 1812 published by Jas Sowerby London.
British Mineralogy
CCCCLXIII
Argilla Topazius

Topaz

  • Div. 1. Crystallized.

The newest subject often attracts to itself the greatest consequence: thus the Scotch Topazes were even figured first in this work, although last discovered. It was, however, their magnitude that gave them that honour. The Cornish Topaz has been known to many for some years, but the crystals are not so readily distinguished, being small and often intermixed with quartz ; they are always found on the gangue, which docs not happen with the Scotch ones. This is an advantage, and points out the geologica situation most perfectly. The crystals of Topaz may be distinguished from those of Quartz by their form, their fracture, and partly by their colour. The crystals are four- or eight-sided prisms, striated on their faces, and terminated by six unequal-sided pyramids; the fracture is perpendicular to their sides and very flat, while that of the Quartz is conchoidal and irregular. The Topaz commonly accompanies crystals of Oxide of Tin; but the present specimen, which has no tin about it, is blackened by the dark-coloured edges of laminæ of Mica. It appears to he part of a vein of secondary Granite, composed of largish grains of Quartz, fine granular Feldspar, small plates of Mica, and minute crystals of Tourmaline. I was favoured with the loan of this specimen by my good friend G. B. Greenough, Esq, M. P. It comes from St. Michael’s mount.

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