When it comes to a mortar and pestle this is the most important question to ask. My primary income comes from working in a compounding pharmacy where I have to mix drugs/chemicals to produce creams, capsules, suppositories, you name it, chances are I've made it.
That being said, mortar and pestles are often completely unusable. For instance, we have wooden ones that customers have given us. Pewter ones etc. When it comes to drugs, just like food, you have to be wary of reactive surfaces. This is why aluminum pans are now almost always lined with steel, or a non-stick surface. Aluminum is highly reactive to acids, so lemon juice, vinegar, even alcohols all get a metallic taste from aluminum.
BIGGUNDOCTOR stated that he was using a cast iron mortar and pestle in his kitchen for grinding spices... that means that every time he grinds those spices, he's adding little bits of cast iron into his diet. Not saying theres anything wrong with that, every time we use a metal spatula on a cast iron skillet we're doing the same thing. He also stated that the pores in the cast iron help to hold onto things. Let me tell you, I absolutely HATE breaking in a new mortar and pestle. The porcelain ones (Wedge-Wood) are much easier to work with when they are new than glass. It takes the glass ones a good 40 uses or so to start getting those scratches that makes them really nice. The only thing he stated that I directly disagree or take fault with is that he said most modern mortar and pestles aren't made correctly, that they're made for pounding. That's sort of the purpose.
Just remember, we're used to seing it in logo's of pharmacy's. That's not to say that the shape we're used to is absolutely correct. A "mortar" comes from the latin Mortarium (spelling) and basically means receptacle for pounding, and "pestle" comes from pistillum (again, spelling) and basically means pounder. So basically, a mortar is any receptacle for pounding an object. Thus a spoon swage is a mortar. And a pestle is any bat shaped object used for pounding. Throw out the rules, step back, have fun with it. Chances are whom ever comissioned the item doesn't plan to use it, but to have it setting on a counter for looks and if thats the case, just make it look CRAZY. The International Journal of Compounding
uses mostly purely decorative mortar and pestles on the cover of their magazine every month. Might be able to find some ideas looking through some of those.