Spinell has been usually found of small size and in red varieties, long since, at Pegu in the Island of Ceylon, but lately it has been found at Aker in Sweden. Specimens from thence are remarkable for being light blue, and larger than those usually brought from Ceylon. The present specimen was reckoned magnificent by my friend Dr. Clarke, who favoured me with the use of it to figure here. I have one purchased at the sale of Fiott’s Minerals, where they are in very confused groups, consequently less distinct, and less fit for a figure. It is found as this specimen, imbedded in carbonate of lime, which shows the rhomboidal fracture, and frequently the diagonals, which are not very common. The green appearance is sometimes sahlite.
The Count de Bournon, so well known as an excellent crystallographer, has discovered colourless transparent spinell on a specimen of his from Vesuvius.
Spinell, which is called by some Balass Ruby, is somewhat commoner than the true or oriental ruby (one of the varieties of corundum), which it often much resembles, but the former has often twenty or more per centum less alumina. The crystal is a regular octaëdron; in general, as the present specimen; but others, chiefly from Ceylon, have many modifications besides the mackle or hemitrope; but of these hereafter. The surface is usually very smooth, lustre considerable; vitreous. It is partly foliated, partly conchoidal. Fragments sharp-edged. Scratches quartz easily; may be scratched by all the varieties of corundum, and consequently may be cut into shape by the lapidary with emery, which is found to consist of corundum, or what has been called adamantine spar, from its being next in hardness to the diamond. Spinell, although inferior to the ruby in hardness, may sometimes vie with it in brightness. All its varieties have been valued in such instances, as among the most precious stones for jewellery.