Notes on “ Inelusions” in Gems, &e. By Isaac Lea. 203 
Opal.—This exquisite gem, which displays such brilliant colours, 
is very highly valued. It is but little harder than glass, and is 
indeed considered as volcanic glass. Its remarkable flashes of 
colour are attributed to fissures, in accordance with the theory of 
Newton’s coloured rings. I have never been able to detect either 
cavities or minute crystals in this beautiful gem, except in two 
cases. One of my specimens has a brown, terminated crystal, a 
six-sided prism of an unknown substance, about one-fifth of an 
inch long, and terminated by a single oblique plane; the other has 
several smaller ones. 
Lapis-lazuwli.—This was used by the ancients as a favourite gem, 
but it 1s not now valued as such. I have not been able to detect 
cavities or minute crystals in any specimen in my possession. 
Corundum.—This very interesting mineral, when in perfect 
transparent crystals, is highly valued as a gem, under the name 
of sapphire, ruby, &c., according to colour. When yellow, it is 
called Oriental topaz; when purple, Oriental amethyst. When 
purely white it is sometimes sold as a diamond. In this country 
we have two localities only of corundum where any large quantity 
has been found, that of Chester County, Pennsylvania, and Frank- 
lin County, North Carolina. From the mines in Chester County, 
several hundred tons have been taken, but no transparent crystals. 
Some opaque ones are bluish and some pinkish. The North 
Carolina locality has produced some very large crystals, and nu- 
merous small ones. Of the latter there have been found many 
quite pure and transparent, and these are sometimes blue and 
sometimes red. But none of them yet found are of value as 
gems. The fine sapphires and rubies are chiefly from Ceylon, 
and they form some of the most beautiful objects in nature. I 
have many of these in the form of worn pebbles, and some in fine 
hexagonal form, as well as hundreds of cut specimens. I have 
examined carefully more than one thousand specimens, with a 
view to discover whatever “ inclusions” they might possess. In 
a communication to the Academy* I described and figured some 
microscopic crystals in these and other gems. Since then I have 
added a very large number to my collection, and among these 
several hundred large and small transparent crystals. In a careful 
microscopic examination of these, I found a large number which 
contain cavities and minute crystals, the former sometimes scat- 
tered irregularly through the mass, and sometimes forming a 
sheet or film. These cavities are of all forms, but usually sub- 
elliptical ; sometimes tubular, and these tubes frequently anasto- 
mose in a very beautiful manner. ‘These cavities are so numerous 
that they frequently give a cloudiness to the specimen, which is 
less valuable as a gem, but most interesting in a scientific point of 
* «Proc, Acad. Nat. Sci.’ 1869, 
VOL. XVII. Q 
