How is the luster of a mineral determined?
By observing how the mineral reflects light from its surface.
There are two different types of luster:
Metallic- containes shiny metals
Nonmetallic- can be described as dull, pearly, glassy, greasy, or silky.
Galena: metallic
http://www.mii.org/Minerals/Minpics1/Galena%202.jpg
Malachite: dull or silky
http://www.emporia.edu/earthsci/amber/go340/images/malachite.jpg