The culinary classic mortar and pestle is crafted veining to easily crush and blend spices and herbs, peppercorns, garlic, nuts, seeds, butters and more. Bruising fresh herbs and mint helps release the natural oils for fuller, more robust flavor. Because marble is a natural product, veining and coloring variations will occur.
Generally speaking, and with some exceptions, I'll discuss further down, these are the qualities in both large and small mortars and pestles that are useful for most common kitchen tasks:
A large pestle:
Just as you wouldn't choose a dinky little toy hammer to drive large nails into wood, you don't want a narrow, skinny, or short pestle when working in a large mortar. If the pestle is too short for the mortar, it's impossible to use it without banging your hand against the mortar's edge. Similarly, if the head isn't broad enough, it will simply push ingredients around instead of pulverizing them.
A stone mortar:
Though there are exceptions, in most cases, you're better off with a solid stone mortar. Stone such as marble or granite is strong, dense, and unyielding, giving you a powerful surface against which to crush things; it's also not nearly as brittle as ceramic, meaning you can really drive some force into it without worrying that the material will crack or chip. Wood, meanwhile, can be a great pestle material in some instances, but is less effective as a mortar. The fact that it has a density and hardness similar to those of many foods we grind makes it less effective—much as diamonds are used to cut glass, we want to use a harder material to crush or grind substances like spices.
An unpolished interior:
Avoid mortars with glossy, smooth bowls, which lack the abrasive qualities crucial to properly shearing and grinding foods.
A round bowl shape:
An evenly round bowl shape makes it much easier to swirl the pestle and grind everything in its path. Cylindrical mortars with sharp corners at the bottom are harder to get into with the pestle, making it more difficult to grind and smash the contents down.
