Use of mortar and pestle is an old style grinding method. Small amount of food sample is placed in cup like structure and pound repeatedly to attain small particles. Using electric blender is the new and current. It often does the job in matter of seconds. It gives good and repeatable results.
However, there are cases when mortar and pestle is more ideal than electric blender. Like pounding garlic and black pepper for daily cooking needs. The mortar and pestle does the job well, easy to clean and saves electricity.
There are three basic types, metal, mineral and wood. Each has their own advantages and weaknesses.
1) Metal – brass, steel, stainless steel. No question about hardness of these metals. They can withstand repeated exposure to pounding force without wear. But be aware that metals react to acid. Always remember not to use it to grind / mash acid foods. Also, steel is prone to rusting. Always keep them clean and dry.
2) Mineral – stone, marble. What happen when two stones hit each other? They have the tendency to break apart. The same goes with these kind of mortar and pestle. Repeated use breaks particles. You might be including stone or marble powder/pieces in your food preparation. Marble is sensitive to acid foods. The latter tend to weaken the first. More particles are going to be included in your next dish.
3) Wood – the lightest among the three types and probably the worse. Wood absorbs water. Long and repeated exposure to food sample make the case worse. Harmful microorganisms loves moist surfaces. You are harboring dangers to food every use.