food-safetymortar-and-pestle
Even after conditioning/seasoning one can expect microscopic chips from using a mortar and pestle.
Intuitively it seems that a metallic M&P would be more inclined to shed not so healthy chips, in contrast to a combination of a hard mortar (granite) and a (food safe) soft pestle (wood) whose chips are non toxic.
There are at least three potential downsides:
Neither.
The traditional instrument for making guacamole, and other Mexican mashed and ground preparations, is the mocaljete, which is similar to a mortar & pestle, but made with a shorter pestle and out of black basalt, a rough volcanic stone:
A regular mortar and pestle out of ceramic or granite will not work for a fat, squishy fruit like avocado; it will just squish around and out of the bowl. You need the rough, pitted texture of the volcanic stone to "grab" the avocado pulp.
Personally, I find that the ideal tool for making guacamole or avocado salsas is actually a regular mixing bowl and a heavy pastry blender:
... but even a large metal serving fork will work fairly well. Better than a regular mortar and pestle will.
Best Answer
A granite mortar and granite pestle; this combination will do everything.
The granite is hard and dense and will not chip under regular usage.
You need to remember that the mortar and pestle must be harder than what you need to grind.
Serious Eats Mortars and Pestles is a good read.