SoMABULA Diamond Field of Rhodesia. 291 



great lake which must once have filled the adjacent portion of the 

 Zambezi basin, and draining an area chiefly occupied by granite and 

 the Archaean banded ironstone. 



The gravels of the Somabula are interesting from their unique 

 lithological character, as far as Rhodesia is concerned, but their chief 

 interest naturally arises from their being the source of various gem 

 •stones, particularly the diamond. The diamonds themselves are 

 peculiar, as almost invariably of a green shade in the rough : this is, 

 however, entirely lost in cutting. They occur in very good crystals, 

 principally octahedra, spinel twins of two octahedra, twinned 

 tetrahedra, twinned hexatetrahedra, dodecahedra, etc. Etched tri- 

 angles are characteristic of the tetrahedral faces. Worn stones are 

 almost entirely absent. The mineralogical associates of the precious 

 .stone are not precisely similar to those of the Kimberley diggings or 

 of the more recent Transvaal discoveries, but they nevertheless present 

 a general resemblance to those of the localities named. Garnets are 

 often common, but are not of the blood-red Kimberley variety. 

 Ilmenitc is uncommon. Both magnetite and haematite, of 

 which grains are numerous, are evidently derived from the Banded 

 Ironstone, while the source of the zircon and of the mica (muscovite) 

 is equally clearly the granite : these minerals have no necessary 

 connection with the original matrix of the diamond. This is also 

 probably the case with the beryls which occur. The typical 

 minerals are (besides the diamond) enstatite staurolite, chryso- 

 beryl, kyanite, and sapphire. Enstatite is the commonest of the 

 minerals popularly grouped together under the name of olivine at 

 Kimberley, but that found here is a brownish variety. Staurolite is 

 an abundant constituent of the sorted material from the puddluig 

 machines. Some of the grains are fairly clear, and might almost 

 be taken for garnets on account of their red-brown colour. Chryso- . 

 heryl is quite abundant for so rare a stone. The prevailing variety 

 is yellow, but the opalescent (precious) " catseye,^^ and the form 

 known as " alexandrite,'' which is green by day and red by candle- 

 light, also occurs. This mineral would seem to be usually a product 

 of contact metamorphism : at the same time, although it is not strictly 

 analogous in a chemical sense, its similarity of composition and iso- 

 morphism with olivine are to be noted. As a gem, the ordinary 

 variety is actually called "chrysolite," one of the names properly 

 applied to olivine. K vanite and saffhire are unequivocal contact 

 minerals, so are rutile and tourmaline , whose occurrence may also be 

 noted. The presence of the first-named is interesting, owing to its 

 softness, which makes its survival rather remarkable. Of the 

 sapphires, both the blue and colourless varieties occur, while true 

 rubies and oriental amethysts are also found, though they are distinctly 

 rare, even for such scarce stones. Another stone whose occurrence 

 may be noted is the so-called "Somabula Blue" ; this is harder and 

 heavier than common bervl, and is biaxial with a wide nphiaxial 

 angle, cleavage flakes shewing the bisectrix normal to them. It 

 seems unquestionably, therefore, a variety of topaz : when cut it 

 is one of the most beautiful gems imaginable. 



