Outward Characteristics of Precious Stones: Lustre

LUSTRE:

"Among minerals great differences are met with as regards the manner in which the light acts upon their surface. In this respect there are two separate effects to be distinguished, lustre and colour, which are one to the other as timbre is to sound in a 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.