Hi digazz.
Traditionally kitchenware is normally made from timbers such as Sycamore, Maple, and so on, the timbers that do not leave a taste in the food. If they are to be used in anger you don't need a finish, just spray them lightly with water and re-sand. Do this a few times and you will have a nice silky finish which can be rinsed after use.
There are food safe finishes around if you want to use them, Chestnut do a food safe oil which has a good reputation.
Some people also use liquid paraffin from the chemist, I have never used it, some even use vegetable cooking oil, the problem with these type of finishes is they need to be continually re-applied after use. Not a good idea IMO if you are giving them as gifts, or for sale.
If the mortar & pestle is just for decorative use the sky's the limit, any wood any finish, just make sure you tell anyone that gets it that it is not for use. You never know these days with the allergies going around.
Hope this helps you a bit.