You probably have one or two already, there are a hundred different styles and types. Some are crocheted, some made with felt, some filled with cotton or yarn, left over from other projects, perhaps, some of the them with polyester fiberfill. Whether you view them as an individual works of collectible art, a fun project to make and sell, or a necessary evil for all serious fabric crafters, seamstresses and quilters, just about everyone will agree, a pincushion (or three!) is a must-have.
A good abrasive mineral is not just hard, it's also tough and sharp. It also must be plentiful (or at least widespread) and pure. My emery grain is natural, milled to the finest grade and mixed with the just the right blend of complementary minerals for pincushion use.
I always recommend using an inner liner for your pincushion, so that if anything happens to your outer covering, the emery doesn't end up going everywhere. This is particularly important if you make and sell pincushions!