132 
POPULAR SCIENCE REYIEW. 
the hardness of quartz, and the specific gravity is about 3. It 
is occasionally used as a gem when of yellow, green, or pink 
colour. This is, however, seldom the case with any of the British 
specimens, w^hich are usually black, like that figured (1) from 
Bovey Tracey, Devon, where fine crystals are associated with 
apatite in granite, but are worthless for ornamental purposes. 
Tourmaline crystallises in modifications of six-sided prisms, 
deeply striated, terminated by three or six planes, and is remark- 
able for its optical properties. Like topaz, it becomes electric 
by heat or friction. It is also found in Scotland and Ireland. 
The well-known gem Topaz (a fluo-silicate of alumina), is not 
uncommon in the granitic rocks of Grreat Britain and Ireland. 
In Scotland magnificent topazes have occasionally been found 
in Cairngorm, and one specimen is stated by Professor Jameson 
to have vreighed nineteen ounces. “ There is a peculiarity in the 
disposition of colour in the Scotch topazes ; their prevailing hue 
is pale blue, but there is a tinge of reddish-brown along the 
acute edges of the prism.” 
The Irish topazes from the granite of theMourne Mountains, 
associated with beryl, are usually colourless, but have sometimes 
a faint pink, blue, or green tinge. Small bluish-white crystals 
are found in the tin veins in the granite at St. Michael’s Mount, 
Cornwall, and at Lundy Island. Topaz crystallises in rhombic 
prisms, longitudinally striated, as shown in fig. 4, has a speci- 
fic gravity of 3*5, and by heat or friction becomes electric, 
like tourmaline. The colourless varieties of topaz, when cut as 
brilliants, are often used to imitate diamonds. 
Lapis lazuli and Turquoise have not been as yet found in the 
British Islands; the only opaque stone of native production 
adapted for jewellery is malachite, or carbonate of copper, which 
is found at Eedruth, Cornwall, the softness of which does not 
entitle it to rank as a gem, though otherwise it is much em- 
ployed as a cheap ornament of personal decoration. Its beauty 
as a stone depends on its concretionary structure, showing zones 
of deep or pale-green colour, as seen in the specimen (fig. 13). 
The various coloured fluor spars of Derbyshire are well known ; 
and in that county also the fibrous variety of gypsum, or alabaster 
(sulphate of lime), called satin-spar, is also worked into beads 
for necklaces, and other similar ornaments, often very pretty, 
especially from the play of light which they exhibit. 
The organic substances, bituminous shale, jet, and amber, 
were used for personal ornaments, as beads and necklaces, 
previous to the Koman colonisation of Britain, as they are 
found not only in the early sepulchral deposits, but in those of 
the Anglo-Koman period in many parts of Great Britain; jet 
being then highly valued, and even an article of export, ac- 
cording to Solinus, at that early period. 
