The Kamfersdam Mine yields diamonds of very inferior quality, dark brown being the predominating colour, and even the majority of the better-class stones from this mine are faintly tinged with brown.
The Kimberley West, formerly known as Theron’s Mine, situated about 30 miles due west of Kimberley, yields a very small percentage of blue-whites, fine “silver capes,” and a large proportion of brown diamonds, somewhat better in quality than Kamfersdam and more regular in shape. The diamonds from this mine present a distinctly “alluvial” appearance, but they are nevertheless distinctive in character from river diamonds and much inferior in quality.
The diamonds from the Leicester Mine are of a distinctive character; they are very much grooved, extremely bad shapes for cutting, and many of the stones are cross-grained.
The Newlands Mine, West Griqualand, about 40 miles north-west of Kimberley, is interesting on account of the occurrence of diamond in what the Reverend Professor Bonney considers to be its true matrix. The workmen occasionally come across well-rounded, boulder-like masses of eclogite, a rather coarsely crystalline rock, sometimes more than a foot in diameter. Some of these boulders have diamonds imbedded in them. One piece examined by Professor Bonney measured approximately 4 inches by 3 inches by 2 inches, and appeared to have been broken off a larger eclogite boulder. In it were seen ten diamonds, mostly well-crystallised octahedra, perfectly colourless, with brilliant lustre, four of them being comprised within a space of a quarter of an inch square. All these diamonds were on the surface. Probably others would have been found inside, but it was not considered desirable to destroy the specimen by breaking it up. It is now in the Natural History Museum, having been presented by the Directors of the Newlands Mine.
Eclogite has been found in other diamond mines, but I am not aware that diamonds have been found imbedded in it except in the Newlands Mine.
Stones from Jagersfontein, in the Orange River Colony, display great purity of colour and brilliancy, and they have the so-called “steely” lustre characteristic of old Indian gems.
Falling off of Yield with Depth
According to tables furnished by the De Beers Company, the yield of the De Beers and Kimberley mines has declined as the depth increases. At the same time the value of the stones has risen, and diamonds are more expensive to-day than at any previous time.
| NUMBER OF CARATS[5] | VALUE | ||
| YEAR | PER LOAD | PER CARAT | |
| s. | d. | ||
| 1889 | 1·283 | 19 | 8·75 |
| 1890 | 1·15 | 32 | 6·75 |
| 1891 | 0·99 | 29 | 6 |
| 1892 | 0·92 | 25 | 6 |
| 1893 | 1·05 | 29 | 0·6 |
| 1894 | 0·89 | 24 | 5·2 |
| 1895 | 0·85 | 25 | 6 |
| 1896 | 0·91 | 26 | 9·4 |
| 1897 | 0·92 | 26 | 10·6 |
| 1898 | 0·80 | 26 | 6·2 |
| 1899 | 0·71 | 29 | 7·2 |
| 1900 | 0·67 | 35 | 10·2 |
| 1901 | 0·76 | 39 | 7 |
| 1902 | 0·76 | 46 | 5·7 |
| 1903 | 0·61 | 48 | 6·3 |
| 1904 | 0·54 | 48 | 11·8 |