musical
instrument. Colour depends upon the nature of the reflected rays,
lustre upon their intensity, and upon certain particular modifications
of their tint which cannot be defined; it depends upon the structure of
the body, its kind of texture, and the greater or less polish of its
surface. Lustre, like colour and transparency, is susceptible of
gradation ; it is more or less vivid, more or less dull; and
disappears entirely in varieties in which the aspect is rough, stony,
or earthy " (Delafosse).
Adamantine lustre.—Intermediate
between metallic lustre and vitreous lustre; it belongs to certain
crystals; to the zircon, and above all to the diamond.
Nacreous or pearly lustre.—A
mixture of silvery and vitreous lustre, resembling, as its name
indicates, the nacre of pearl. Certain varieties of corundum possess
this lustre in a very pronounced manner.
Silky lustre.—Due to straight fibres disposed very closely and of equal thickness. It resembles the sheen of certain fabrics of mohair.
Oily lustre.—The
stones which possess this lustre are generally vitreous stones, which
always seem, even when newly fractured, to have been impregnated with
oil.
Resinous lustre.—A medium between the oily and the vitreous lustre. The opal generally presents this aspect
