A rich blue mineral, the Lapis-lazuli of jewellery (34b), brought from Persia, Siberia, Bokhara, and Chili, is a mixture of various species. When powdered, Lapis-lazuli furnished the once costly pigment ultramarine; but by the discovery of a method of producing an artificial and cheap form of the latter, the use of the mineral as a pigment has ceased.
The sulphates of Strontium, Celestite (35c), of Barium, Barytes or Heavy Spar (36a), and of Lead, Anglesite (36e), are all represented by series of specimens.
Gypsum, or Selenite (36f), is a hydrated sulphate of the metal Calcium: when heated, it gives up its water of crystallisation and falls to a white powder, known as “Plaster of Paris,” which, when moistened, again combines with water and yields a coherent solid. Gypseous Alabaster, a massive variety of Gypsum (36h), owing to its whiteness, fine texture, and softness is used as a material for statuettes and other indoor ornaments. Oriental Alabaster is a harder substance, Stalagmitic Calcite, carbonate of Calcium.
Borax (37c), a borate of Sodium, is much used as a flux, also in soldering, and in the preparation of easily fusible enamels. It was formerly carried over the Himalayas on sheep and goats from lakes in Tibet, but is now obtained largely from the Borax-lakes of the United States, and is also extensively prepared from the boracic acid lagoons in Tuscany.
Nitratine or Soda-nitre (37d), found in Chili in beds of large extent, is largely used for the preparation of Nitric Acid and Saltpetre, and is also used as a fertiliser.
Calaite or Turquoise (38g), a phosphate of the metals Aluminium and Copper, is generally massive; only very rarely does it occur in the crystalline state. Being as hard as Felspar and taking a good polish, it has been much prized in jewellery; that which comes into the market is chiefly brought from the Turquoise-mines not far from Nishapur, in Persia.
As a supplement to the collection of simple Minerals, is arranged, in case 41, a group of natural substances which either belong or are closely related to the Mineral Kingdom, although, in the formation of most, organised matter has played a very important part. The most important of these are Coal and Amber. Coal (41a), in most of its varieties, gives structural evidence of its vegetable origin: its chemical composition depends on the amount of change which has taken place, and thus is less definite than in the preceding minerals. In the variety called Anthracite all traces of the original organised structure have disappeared. Amber (41c), in ancient times regarded as one of the precious stones, is likewise of vegetable origin. It is fossil resin, chiefly derived from trees allied to the pines; its originally sticky condition is proved by the insects sometimes found enclosed.