PRECIOUS STONES 185
is
not clean, shows a black or purple tendency, whereas the spinel
inclines to a suspicion of yellow. When the color of the latter is most
free from the yellow taint, it still lacks the lustre of the ruby. The
dichroiscope will establish their identity, as the ruby is doubly
refractive and the spinel single.
Hardness. Specific Gravity.
Ruby.............9 3.9 to 4.1 Refraction double.
Spinel............8 3.60103.63 Refraction single.
4. Ruby and Garnet
Some
of the finer red garnets have a very clear and beautiful red color,
and when finely cut might deceive some. They all lack the hard lustre
and transparent color of the ruby, and are singly refractive. The
difference can be found in the hardness. Specific gravity is not as
sure a test, as some garnets are nearly as heavy as the ruby.
Hardness. Specific Gravity.
Ruby...........9 3.9 to 4.1 Refraction double.
Garnet.........7.25 3.65103.86 Refraction single.
5. Spinel and Garnet
There
is a greater resemblance betwen these two than any other red stones,
especially when cut alike. Some of the garnets are very nearly spinel
color, and, as they are both single refraction stones, and their
specific gravity and hardness are very nearly the same, it is difficult
to decide between them, except by very careful tests. There is,
however, usually a clearer, finer appearance to the body of the spinel,
and a difference in the hardness can be detected. The spinel and some
of the essonite variety are most alike.
Hardness. Specific Gravity.
Spinel.................................8 3.60 to 3.63
Essonite...............................7.25 3.65
