582 DETERMINATION OF PRECIOUS STONES 



but tig-er-eve is of a bright golden yellow colour and hawk's-eye dark blue. Cat's-eyc never 

 exhibits the magnificent golden lustre characteristic of tiger-eye, the light reflected from it 

 being more milky and opalescent in character. 



The sheen of chatoyant obsidian is always less brilliant than that of the other stones 

 of this group, from which it is also distinguished by a lower density (sp. gr. = 2'5 — 2"6), 

 and a lower degree of hardness (H = 5|). 



3. Stones with a metallic sheen. — Under this head come hypersthene, bronzite, 

 bastite, and diallage, all belonging to the pyroxene group of minerals. Their hardness is 

 rather less than 6, and their specific gravity varies between 3'3 and 3"4. The metallic 

 reflection of hypersthene is copper-red in colour, that of bronzite is bronze-yellow, green, or 

 brown, while that of bastite and diallage ranges from green to brown. Hypersthene is 

 readily identified, but it is less easy and of but little practical importance to distinguish 

 between the remaining stones. Their distinguishing features have been pointed out in the 

 special description of these stones. 



4. Stones flecked, "with metallic shining points. — Avanturine-quartz and 

 sun-stone (avanturine-felspar) are flecked with specks exhibiting a red metallic sheen. They 

 are distinguishable by their difference in hardness, the hardness of avanturine-quartz being 

 7 and that of sun-stone 6. The artificial avanturine-glass is a close imitation of these 

 stones, the metallic reflection being given by the enclosure of small octahedra of jnetallic 

 copper. These crystals, with their regular triangular faces, are distinctly observable when 

 the glass is examined with a lens, and their presence serves to distinguish the imitation 

 from the genuine stone. Avanturine-quartz possessing all the essential characters of the 

 ordinary red variety, but of a green or blue colour, occurs as a rarity. 



5. Stones exhibiting a play of variegated colours. — Precious opal, iridescent 

 quartz, labradorite. Opal is usually white, less commonly yellow or red, rarely black ; the 

 play of variegated colours extends over areas of varying size. It has a hardness of 6 and a 

 specific gravity which ranges from 1"9 to 2"2, features which serve to distinguish it from 

 water-clear, iridescent quartz, with a hardness of 7 and a specific gravity of 2'65. 



Labradorite is dark grey; the play of colour is confined to one surface, and frequently to 

 straight bands running across this surface. It has a hardness of 6 and a specific gravity 

 of 2'7. In rough pieces of labradorite a perfect cleavage in one direction is always 

 noticeable. It would be impossible to mistake labradorite for opal, or vice versa, but it is 

 possible to confuse precious opal and iridescent quartz, although the characters mentioned 

 above are sufficient to distinguish one from another. Labradorescent felspar is very similar 

 to labradorite, but its play of colour is less fine. 



