dges between r, s and m. There are perfect
cleavages parallel to the faces of a rhombohedron which truncate the
polar edges of r: from the cleavage cracks internal reflections are
often to be seen in the crystal, and it was on account of this that the
mineral was named dioptase, by R. J. Hauy in 1797, from [Greek:
diopteuein], "to see into." The crystals vary from transparent to
translucent with a vitreous lustre, and are bright emerald-green in
colour; they thus have a certain resemblance to emerald, hence the early
name emerald-copper (German, _Kupfer-Smaragd_). Hardness 5; sp. gr. 3.3.
The mineral is decomposed by hydrochloric acid with separation of
gelatinous silica. At a red heat it blackens and gives off water. The
fine crystals from Mount Altyn-Tube on the western slopes of the Altai
Mountains in the Kirghiz Steppes, Asiatic Russia, line cavities in a
compact limestone; they were first sent to Europe in 1785 by Achir
Mahmed, a Bucharian merchant, after whom the mineral has been named
archirite. More recently, in 1890, good crystals of similar habit, but
rather darker in colour, have been found with quartz and malachite near
Komba in the French Congo. As drusy crystalline crusts it has been found
at Copiapo in Chile and in Arizona.

[Illustration]

Dioptase has occasionally been used as a gem-stone, especially in Russia
and Persia; it has a fine colour, but a low degree of hardness and the
transparency is imperfect. (L. J. S.)

DIORITE (from the Gr. [Greek: diorizein] to distinguish, from [Greek:
dia] through, [Greek: oros], a boundary), in petrology, the name given
by Hauy to a family of rocks of granitic texture, composed of
plagioclase felspar and hornblende. As they are richer in the dark
coloured ferromagnesian minerals they are usually grey or dark grey,
and have a higher specific gravity than granite. They also rarely show
visible quartz. But there are diorites of many kinds, as the name
applies rather to a family of rocks than to a single species. Some
contain biotite, others augite or hypersthene; many have a small amount
of quartz. Orthoclase is rarely entirely absent, and when it is fairly
common the rock becomes a tonalite; in this way a transition is
furnished between diorites and granites. It is rare to find the pure
types of "hornblende-diorite," "augite-diorite," &c., but in most cases
the rocks contain two or more ferromagnesian silicates, and such
combinations as "hornblende-biotite