Sulphuret of Iron

This singular formation of Pyrites, although of old in my collection, is, I believe, new in description. Crystallographers say very little of the concave or convex rounding of crystals of any sort, although it is often a strong characteristic of some substances, especially the convex, as in the Diamond:—Crystallography, part 2d, and Galæna, tab. 131 of this work, &c. The concave is, however, much more rare: indeed the appearance is so extraordinary, that it has been looked at more with an eye of wonder than investigation. The present specimens are examples of this concave formation, and of the tarnish and iridescence which partly disguise substances. They are nearly the same within, although the smallest is so light-coloured outside, and has a yellow, red, and blue iridescence; and the other nearly a fair yellow metallic tarnish. This latter specimen also shows the convex formation, which is so great a contrast to the former that it is altogether a wonderful group, whether caused by the solvent or quantity of material attracted for aggregation. It seems to be an accumulated repetition of this formation that represents the Cockscomb of the following table. We have, however, still some very valuable and extraordinary specimens that primarily lead to this peculiarity, of which we shall be happy to make observation, as they are truly instructive.