Soft pastels consist of pure powdered pigments, which are the same pigments used for all art mediums, such as watercolours, oil paints, gouache etc.
Pastels are then made by mixing dry pigment, the least amount of a neutral hue binder and some chalk, to form a thick paste. The paste is then formed into sticks and allowed to dry.
Soft pastels do crumble but are still usable in smaller pieces.
Some pastelists start by breaking the pastel stick in two.
Soft Pastels have high permanence ratings, which means that they won't fade over time.
No other medium allows you to apply rich colour to the support as quickly as you can with pastels.