Agricola in 1546,
and is from the German _blenden_, to blind, or deceive, because the
mineral resembles lead ore in appearance but contains no lead, and was
consequently often rejected as worthless. Sphalerite, introduced by E.F.
Glocker in 1847, has the same meaning ([Greek: sphaleros], deceptive),
and so have the miners' terms "mock ore," "false lead," and "blackjack."
The term "blende" was at one time used in a generic sense, and as such
enters into the construction of several old names of German origin; the
species under consideration is therefore sometimes distinguished as
zinc-blende.

[Illustration: Fig. 1. ]

Crystals of blende belong to that subclass of the cubic system in which
there are six planes of symmetry parallel to the faces of the rhombic
dodecahedron and none parallel to the cubic faces; in other words, the
crystals are cubic with inclined hemihedrism, and have no centre of
symmetry. The fundamental form is the tetrahedron. Fig. 1 shows a
combination of two tetrahedra, in which the four faces of one
tetrahedron are larger than the four faces of the other: further, the
two sets of faces differ in surface characters, those of one set being
dull and striated, whilst those of the other set are bright and smooth.
A common form, shown in fig. 2, is a combination of the rhombic
dodecahedron with a three-faced tetrahedron y (311); the six faces
meeting in each triad axis are often rounded together into low conical
forms. The crystals are frequently twinned, the twin-axis coinciding
with a triad axis; a rhombic dodecahedron so twinned (fig. 3) has no
re-entrant angles. An important character of blende is the perfect
dodecahedral cleavage, there being six directions of cleavage parallel
to the faces of the rhombic dodecahedron, and angles between which are
60 deg.

When chemically pure, which is rarely the case, blende is colourless and
transparent; usually, however, the mineral is yellow, brown or black,
and often opaque, the depth of colour and degree of transparency
depending on the amount of iron present. The streak, or colour of the
powder, is brownish or light yellow, rarely white. The lustre is
resinous to adamantine, and the index of refraction high (2.369 for
sodium light). The substance is usually optically isotropic, though
sometimes it exhibits anomalous double refraction; fibrous zinc sulphide
which is doubly refracting is to be referred to the hexagonal species
wurtzite. The specific gravity i