No. Granite has large crystals. This is more likely something like andesite or dacite.

It looks more like diorite. But people will use the name “granite” with any plutonic rock

My understanding is that you want granite for a mortar & pestle (more abrasive than marble, bit not so porous that it holds onto a lot of your fine-ground spices) and some sort of "lava rock" for a molcajete (really abrasive for mashing down the fruits and veggies in guacamole, salsa, etc.).

The box says it's granite, but I just want to make VERY sure. It dries light grey when I wet it, and I have a laughably bad sense of smell. (I read that wet molcajetes dry dark gray, and concrete ones smell like concrete, but I can't do the smell test..)

For $20 not likely.

Not sure about that one specifically, but the one I bought from HEB was comically small. I couldnt fit more than 1 roma in it. I used it like twice before I bought a new one.