Violet Schorl, or Thumerstone

I do not know of this substance being found in Great Britain, excepting in the parish of St. Just in Cornwall. It is found in many places abroad, as in Siberia, Norway, Dauphiny, and at Thum in Saxony, whence its name Thumerstein, it having been first discovered at that place.

We find it crystallized in very oblique rhombs—see the right hand lower figure. Its primitive is a rhomboidal prism, similar to that of the Sulphate of Barytes—see the left hand figure. It is mostly of a dull purplish colour, whence its name Violet Schorl; though it is sometimes blueish, brownish, or grayish.

In all the British specimens the crystals are confusedly grouped and small, the foreign ones being often as large as the geometrical outlines. The fracture is small, rather irregularly conchoidal, and has a glassy appearance. Thumerstone is more or less transparent, breaks easily, is rather brittle, may be scratched with difficulty by a knife, and melts easily without addition by the blowpipe into a lightish green glass. Spec. Grav. 3.113–3.300.

Analysis by Vauquelin:

The crystals are found in hollows in an earthy matrix of itself, often having a fine Asbestus covering them which looks like Clay, but may be easily detected by being wetted, as it is composed of extremely fine fibres. I find also some crystallized Carbonate of Lime with the rhombic fracture.

I am obliged to Mr. Penneck; Mr. Trevillian, and Mr. Rashleigh, for specimens.