Overgrowths on Diamond. 
99 
much cracked. Naturally it is only in the broken specimens that the 
structure is revealed. The condition of the specimens indicates a higher 
co-efficient of expansion for the cement than for diamond ; unless the cement 
is subject to secular shrinkage, which on various grounds is not altogether 
improbable. The colour of the cement varies from light to dark dirty-grey. 
It is much softer than diamond, writing a light grey streak on black 
amorphous bort. The denser pieces have a somewhat higher specific gravity 
than pure diamond ; a lower specific gravity suggests porosity. Like 
diamond, the cement is insoluble in acids. 
On account of the theoretical importance of these hailstone-like over- 
growths I venture to attempt a description of some characteristic forms. 
The majority are from Bultfontein. 
1. A broken piece showing an irregular core of light grey cement^ 
surrounded by a thin shell of diamond, then a thin shell of cement, 
the whole enclosed in a thicker shell of diamond of irregular outline. 
Weight 4*5 carats. 
2. An irregular lump showing no trace of crystal diamond. A crater- 
shaped hole in a projecting corner contains a tiny fragment of bright 
green mineral, probably chrome diopside. 0*5 carat. 
3. A broken piece with a dark grey cement interior surrounded by . 
a confused mixture of light grey cement and diamond. 0*7 carat. 
4. A flat piece showing a centre of clouded crystal diamond sur- 
rounded by an inner ring of black crystalline diamond, and an outer 
ring of light grey cement. The original stone, of which this is a 
fragment, must have been roughly spherical. 0*5 carat. 
5. A flat cleavage fragment showing a hexagonal flake of clouded 
and spotted diamond with an outer crystalline shell of a composition 
apparently intermediate between diamond and cement. This specimen 
is of further interest as showing dodecahedral cleavage with the dode- 
cahedral striations. 1 carat. 
6. A flat cleavage fragment in plan something like the section of a. 
pear cut parallel to the stalk. This has a crystalline core surrounded 
by cement, then a thin crystalline shell, then a thicker shell of cement 
intersected by a portion of a very thin crystalline shell following the 
contour of the first for about a quarter of the way round its perimeter, 
and lastly, a crystalline casing. 0-2 carats. 
7. A broken-off corner consisting of a black crystal interior sur- 
rounded by alternate shells of cement and diamond. Three more or 
less complete crystal shells and four cement ones may be counted. 
0*2 carat. 
8. A flat fragment consisting of a crystal central portion surrounded 
by four concentric shells of about 0*5 mm. thick, two of light-coloured 
cement and two of crystal. 3 carats. 
