Definition of lustre

i. The character of the light reflected by minerals, it constitutes one of the means for distinguishing them. There are several kinds of lustre: metallic, the lustre of metals, adamantine, the lustre of diamonds; vitreous, the lustre of broken glass, resinous, the lustre of yellow resin, as that of eleolite, pearly like pearls and silky, like silk. These lustres have different degrees of intensity being either splendent, shining, glistening, or glimmering. When there is a total absence of lustre, the mineral is characterized as being dull.

Ref: Fay

ii. In ceramics, a glaze, varnish, or enamel applied to porcelain in a thin layer, and giving it a smooth, glistening surface.

Ref: Standard, 2