100 



MINERALS or ECONOMIC VALUE, 



LOCALITIES OTHER THAN NEW SOUTH WALES. 



These show-cases should be studied from left to right. 



GEMSTONES.* 



Case 78 South Africa. — In the first column on tlie left are diamonds 



Diamond. and models, the largest of the latter representing a stone weigh- 

 ing 103|- carats. 



5962, a stone weighing I'i carats, represents one of the most 

 characteristic forms assumed by diamonds crystals — an eight- 

 sided double pyramid {octahedron'^) . 



2343, a not uncommon flattened form of tetrahedron (3 carats). 



5961, a very rem ark able specimen (3i carats), consistingof three 

 cubes intergrown. It is rare to find the diamond crystallising in 

 the form of the cuhe. The edges of the cule are modified by a 

 series of overlapping faces of the octahedron, giving rise to an 

 appearance of bevelling such as would result from the presence 

 of the rhomhic dodecahedron. The stone is grey in tint, 

 measuring 5 mm. in length of edge, and weighing 3^ carats. 



2345, carhonado. 



2346, a most interesting specimen, showing a small crystal 

 embedded in the so-called "blue ground" in which the Kimberley 

 stones are found. The crystal has the usual pyramid shape : 

 the striations on the faces should be noticed. t 



The nature of the country, and of the material in which the 

 diamonds are found, is well shown by a good series of specimens, 

 and by descriptive labels and diagrams at the head of the show- 

 case. The stones are found scattered through a decomposed 

 material occupying oval pipes vertically traversing the basalts, 

 shales, and quartzites of the country. This material is known 

 as "blue ground." 



* As the diamond-bearing: material is now criislied immediately on coming from the mine, 

 t is verj' unlikely that specimens such as this will be forthcoming in the future. 

 "f For remarks about diamonds, and notes on New South Wales gemstones, see page 15. 



