MR. .T. T. IIOTBLACK ON PRECIOUS STONES. 
31 
a hardness of 6.5 to 7.0, and 
composition is variously stated : 
a S.G. 
of 2.25 
to 2.7 
Silica 
84.0 
83.0 
69.5 
Protoxide of Iron 
5.0 
6.0 
2.5 
Alumina 
4.6 
3.0 
2.6 
Soda 
3.5 
5.5 
5.0 
Lime 
2.4 
2.5 
7.5 
Potash 
— 
— 
7.0 
Magnesia ... 
— 
— 
2.6 
It is never crystallized, is generally of a dark colour, and almost 
opaque. It is sometimes used for cheap jewellery. 
Paste or Strass. 
I have some hesitation in including this substance in a description 
of precious stones. Paste should be a specially fine kind of glass 
manufactured expressly for artificial gems. 
Stras should be a particular kind of paste, so named after its 
inventor, Joseph Strasser, a German. But I have seen Strass or 
Stras described as a volcanic rock from the Rhine, much used in 
the making of artificial gems, and I have reason to suspfect, both 
from information derived from those connected with the trade, and 
also from the examination of specimens of so-called paste, that 
much of it is really an impure form of rock crystal, perhaps the 
same or a similar stone to that described by Thompson, under the 
name of Kilpatrick quartz, which he thinks entitled to rank as 
a separate species, with a hardness of 7, a S.G. of 2.5, and 
a composition of silica 96.0, water 3.0. This is a hardness 
unattainable, I believe, in glass, and agrees with specimens of 
“ paste ” with which I have met. 
