How to choose ... a mortar and pestle
Mortars and pestles seem old-fashioned compared with the electronic equipment that has (mostly) replaced them, but there are some jobs for which they are irreplaceable.
What are they? They come - or should come - as a pair; you can't use one without the other. The pestle is used to grind, crush or pound ingredients in the bowl-like mortar. They were originally used by apothecaries.
What are they made of? Marble, porcelain, volcanic rock, ceramic, granite and hardwood.
What are the differences? There are advantages and disadvantages to all of them. Hard materials such as marble and granite are good because they're non-porous, so they don't absorb the smell of the ingredients being ground. Mortars made from volcanic rock need to be seasoned to smooth out the rough interior. Wooden mortars and pestles are inexpensive, but because they're porous, they absorb strong odours and can warp or split.
What to look for? Size and weight. Small mortars are fine if you want to grind seeds, but if you want to make mayonnaise or pound a lot of shrimp heads for a bisque, you'll need larger types. Heavier mortars are more stable. Some mortars are etched on the interior, which gives a rough surface that's better to crush seeds upon.