http://www.phisick.com/images/apothmortar-and-pestle-brass-101.jpg
By going to Taverly. On the minimap, you'll see a herblore shop. There he will sell it for a few coins.
It is a small bowl and grinder used to crush/pulp medication, herbs and spices as well as other things.
we clean it with the help of accetone
The best use is definitely to crush fresh Ligurian basil (together with garlic, oil and pecorino cheese) in order to make the "pesto", a green sauce to be used for pasta's first courses....
I think they are used to grind ingredients.to crush a substance into powder