The mortar and pestle set is a handy tool that many Pagans - and other folks - use for grinding and blending herbs and dried ingredients together during a magical working. The set consists of two pieces - the mortar, which is typically a bowl, although it can be flat as well, and the pestle, which is held in the hand.
Place your herbs, spices, or other dry goods into the bowl and hold it steady with one hand. Using the other, hold the pestle. By pressing the pestle down into the mortar, and moving it back and forth, you can grind and blend the herbs or other items for spellwork. This is a great tool to use if you're using dried herbs which may be in large pieces. It also works nicely with fresh herbs, because the grinding motion of the pestle will help release essential oils from the leaves.
Mortar and pestle sets come in a variety of materials, and you can usually find one in your local kitchen supply store. They are available in porcelain, wood, marble, and even metal. In South America, a large porous stone called a molca jate is used to grind grain and vegetables - if you're working with large items such as corn or wheat, consider using one of these instead of the smaller mortar and pestle.