Luster

Luster is the effect the surface of the coin has at reflecting light off of the coins surface as the coin is moved.

Most silver coins that have not been cleaned would have bright white surfaces with very light lines originating from the center of the coin outwards toward the rim which is know as cartwheel luster.

The lines are caused by the stretching of the metal as the coin is struck by the dies (if I recall correctly)

marcus,

Luster is created from the dies coming together under tremendous pressure. As the metal flows through the dies it leaves a "frosting" on the surface.

If you have an uncirculated coin, hold it at a 45 degree angle to a light source and slowly rock it back and forth. You'll notice the frosted area easily. Now find an AU coin that's seen some mileage and do the same. You'll note that it's been smoothed (frost removed)on the highest points of the design.