17 



Oxide of aluminium, A1203.* Sapphires are common enough Case 49. 

 in certain river-drifts in New England and elsewhere, where it ^^^ ^^^* 

 is associated with zircon, topaz, &c , but it is seldom that stones 

 suitable for jewellery are found. Several cut stones are exhi- 

 bited. Next to the diamond, oxide of aluminium is the hardest 

 substance found in Nature (11 = 8). The density is high (specific 

 gravity about 4), so that it quickly sinks in heavy liquids. (See 

 page 15.) Occasionally pebbles will be found showing some- 

 thing of the original crystalline form — a six-sided double 

 pyramid when perfect, breaking by transverse cleavage into six- 

 sided plates. A crystal of sapphire from Tumbarumba, showing 

 a flat termination, will be found in Students' case S3. 



Silicate of zircon ia : The gemstones hyacinth Siwdi jargoon are Zircon, 

 varieties of zircon. It is a hard mineral (H=:7'5) of high 

 density, with a lustre so brilliant that, when colourless, it may 

 easily be mistaken for the diamond. It varies in colour from red- 

 brown, to colourless, but the colour is permanently destroyed by 

 heating ; a tray of stones thus rendered colourless is exhibited. 

 Owing to its high density (nearly twice that of quartz) it cannot 

 be readily separated from gold by panning, so that the gold in 

 zircon sand cannot be freed by concentration. Zircon is common in 

 many drift deposits. (See page 22.) "When good crystals are 

 found they can be readily recognised by their square sections 

 (7635). Most commonly, however, zircon occurs as rounded 

 grains. 



\ 



Simple crystal of Zircon. Prism v,ii\i jpyramid. 



* Oxide o! aluminium, or alumina, in its common massive form, is known as corundutn. 

 (See page 154, and show case 99.) 



