Lustre which can also be spelt luster, is a description of the way that light interacts with the surface of a crystal, rock or mineral. It's the reflection or scattering of light from the surface of the stone and words used to describe the type of lustre that's being displayed varies from metallic to vitreous to resinous to earthy. In order for something to have a high lustre it needs to have a relatively smooth surface and high reflectivity. Polishing will often improve lustre.
The term lustre is sometimes confused with brilliance which should if used correctly, describe the reflection of light from within a gemstone that has been faceted.
There are two broad types of lustre, metallic and non-metallic and most crystals rocks and minerals will fall into one category or the other. Precious metals usually have a metallic lustre whilst gemstones generally tend to be non-metallic. Minerals can vary however and whilst some like hematite and pyrite are metallic, the vast majority are non-metallic.
The word lustre comes from the Latin 'lux' meaning 'light' and there are many terms used to describe it. Those which relate primarily to polished minerals and gemstones include vitreous, dull, waxy, pearly, silky, resinous, greasy, earthy, metallic and adamantine. Lustre describes the appearance of a crystal rock or mineral in reflected light.