I have 4 M&Ps and a spice grinder:
The first one on the left is hewn roughly from granite and I got it for about $20 from a province here that specializes in M&Ps. (Thailand is a land of spices and curries and they take M&Ps very seriously.) This one is heavy, beautiful, and outstanding for grinding anything, whether dry or wet. Garlic turns into pulp in no time with little effort. The pestle is very heavy with a large rounded but still rough bottom.
The second one is made of earthenware. It is tall with a well-fitting wooden pestle for ease of crushing and prevention of matter/ingredients flying out during the tamping. It is perfect for making spicy Thai papaya salad.
The third is tiny but pretty heavy for easily crushing small amounts of herbs and spices. Made of stone.
The fourth is a German-made stainless steel set and I use it specifically for grinding small amounts of saffron. The smooth finish ensures no precious saffron bit is wasted or clings to the mortar.
I also have a Braun coffee grinder dedicated to grinding spices.
'Never eat more than you can lift.' - Miss Piggy