SATIN-SPAR 527 



lapis-lazuli. The supply of malachite in small pieces is always more than equal to the 

 demand ; the price of such material is therefore low, but large compact masses of fine colour 

 are rare and command much higher prices. 



The general appearance of malachite is so characteristic and pecuhar to itself that the 

 mineral can scarcely be mistaken for any other stone. Green chrysocolla, the so-called 

 " siliceous malachite," sometimes resembles malachite, but can be distinguished from it by 

 the fact that it does not effervesce when a drop of hydrochloric acid is placed upon it, 

 while malachite, being a carbonate, does do so. This test may be applied to a cut stone 

 with no serious damage, provided the drop of acid is placed on an inconspicuous part and 

 quickly wiped oft". 



CHESSYLITE. 



The chemical composition of this mineral is very similar to that of malachite, 

 but the constituents are present in different proportions, the chemical formula being 

 HgO.SCuO.^COg. It has a fine dark blue colour like that of lapis-lazuli, a character to 

 which its other name, azurite, has reference. Its specific gravity is 3'8, and its hardness 3f , 

 so that it is both denser and softer than lapis-lazuli, from which it may also be distinguished 

 from the fact that it effervesces with acid like malachite. In mass, chessylite is scarcely 

 ever perfectly transparent, being at best only translucent. Its lustre is vitreous and the 

 mineral takes a good polish. It is used to only a very limited extent. 



SATIN-SPAR. 



The term satin-spar embraces certain finely fibrous varieties of three distinct mineral 

 species, which are usually white in colour and possess in common a satiny or silky lustre. 



Calcite. — This mineral sometimes occurs as veins with a finely fibrous structure, 

 the fibres being arranged perpendicularly to the two parallel walls of the vein. When the 

 plate-like masses from such a vein are broken across, the fractured surface shows a fine silky 

 lustre which can be increased by polishing. This variety of satin-spar is, therefore, 

 sometimes cut for beads, or ear-rings, or similar ornaments. A cut and polished convex 

 surface shows a band of chatoyant light, as in cat's-eye, which is due to the same cause, 

 namely, the finely fibrous structure of the mineral. Polished objects very soon become 

 scratched and disfigured, for the mineral has a hardness represented by 3 only. Some 

 of the best material is obtained from near Alston in Cumberland, where it occurs in 

 straight regular veins measuring 2 or 3 inches across in a black shale of Carboniferous age. 

 It has a snow-white colour with sometimes a delicate rosy tinge, and the satiny lustre is 

 very conspicuous on polished surfaces cut parallel to the fibres. 



Marble is a granular, crystalline aggregate of calcite, but as this variety has no 

 application as a precious stone it need not be considered further. Onyx-marble from 

 Tecati in Mexico and onyx-alabaster from Egypt are varieties of stalactitic marble, which 

 in the majority of cases also consist of calcite. 



