Crushed or naturally eroded emery (known as black sand) is the best abrasive for the scale modeling details of you miniature pieces. It can be used for grinding glass, and is still used in jewelry making for a highly reflective finish. However, this does take some skill. A variety of shapes and sizes make these sanding sticks the ideal choice for adding the desired finish and final details to your workpieces of any size and material.
The sanding material (emery) largely consists of the mineral corundum (aluminum oxide), mixed with other materials such as the iron-bearing spinels hercynite and magnetite, and also rutile (titania). Industrial emery may contain a variety of other minerals and synthetic compounds such as magnesia, mullite, and silica.