STONES WITH SPECIAL OPTICAL EFFECTS 581 



gravity and small degree of hardness ; and can be identified unmistakably by the fact that 

 it effervesces when touched with hydrochloric acid. The green solution of the mineral thus 

 obtained imparts a blue colour to the flame of a spirit lamp. 



Riband-jasper is opaque, and is characterised by a banded structure, the bands being 

 straight, and coloured alternately green and brownish-red. Agate is always more or less 

 translucent, the bands of which it is built up show great variety of arrangement and marked 

 contrast in cdfeur. 



Heliotrope is the name given to dark green chalcedony (plasma) spotted 

 with red. Amber sometimes exhibits spots and clouds of a brown or yellow colour; 

 it is distinguished from the other stones by the characters enumerated in preceding- 

 tables. 



Glass of variegated colours is sometimes met with, but is not likely to be mistaken 

 for any of the stones included in this group. 



8. STONES WITH METALLIC LUSTRE. 



This group comprises iron-pyrites and hfematite (including iserine) : these minerals are 

 respectively yellow and fjlack in colour, and have been dealt with in Tables 5 and 4. 



C. STONES WITH SPECIAL OPTICAL EFFECTS. 



These optical effects are so characteristic of the stones which exhibit them that the 

 consideration of other distinguishing features is almost superfluous. 



1. Stones exhibiting a chatoyant star. — Star-sapphire, star-ruby and "star- 

 topaz." These all belong to the species corundum and are generally refeiTed to as star- 

 stones or asterias. Having a hardness of 9 they are capable of scratching topaz. 

 The specific gravity of each is about 4. They are respectively blue, red, and yellow in 

 colour, while in other features they are identical. 



2. Stones exhibiting a chatoyant band. — Girasol-sapphire, girasol-ruby , 

 " girasol-topaz," adamantine-spar ; cymophane, cafs-eye, tiger-eye, and hawk's-eye ; moon- 

 stone and chatoyant obsidian. 



The four first mentioned stones, like those under the preceding division, belong to the 

 mineral species corundum, and possess all the features characteristic of that mineral, 

 including great hardness. Adamantine-spar is characterised by its small degree of trans- 

 lucency and its hair-brown colour, features which distinguish it from the other three stones, 

 which are only slightly clouded. The remaining stones are all softer and specifically lighter 

 than corundum. 



Cymophane or oriental cat's-eye is very similar in appearance to ordinary quartz-cafs- 

 eye, but differs from it both in hardness and specific gravity, the former being 8| and 7, and 

 the latter 3 '7 and 2'65 in the two minerals respectively. The greater lustre and transparency 

 of cymophane are usually sufficient to distinguish it from quartz. 



Moon-stone resembles cymophane in the possession of a band of chatoyant light, but 

 differs from it in being colourless and almost transparent, much lighter (sp. gr. = 2'6) and 

 softer (H = 6). 



Tiger-eye and hawk's-eye, like quartz-cat's-eye, have the characters of quartz, namely, 

 a specific gravity of 9,65 and a hardness of 7. Both possess a markedly fibrous structure, 



