PRECIOUS STONES. 



Diamonds, spinel ruby, or garnet are never found as six- 

 sided prisms, and thus several commoner crystals can be 

 distinguished from them ; nor are emeralds, sapphire, zircon 

 found as cubes, octahedrons, or rhombic dodecahedrons. 

 With the exception of diamond (which is pure carbon), pre- 



Fio. 47. 

 ONE COMMON FORM OF GARNET. 



FIGS. 48 and 49. 

 SOMB FORMS OF SAPPHIRE. 



cious stones may be divided into two classes those which 

 have alumina as the base, and those which have silica. Of 

 the first are the sapphire, ruby, emerald, &c. ; of the second 

 are the amethyst, opal, cat's-eye, agates, &c. 



To estimate the value of an uncut diamond there is no 



Fro. 49A. 



ONE FORK OF CRYSTAL OF TOPAZ. 

 The crystals often have terminal facets. 



FIG. 50. 

 ONE COMMON FORM OF BERYL. 



The crystals often have bevelled 

 terminal edges. 



fixed rule, on account of the fluctuation of prices. 



The hardness and lustre are the most reliable tests to 

 detect this choicest of all gems. A diamond will scratch 

 any mineral ; but in testing it care has to be taken that the 

 angles be not broken, as, notwithstanding hardness, it is 

 rather brittle. 



