There are two factors you want to look at when choosing a stone for this purpose: thermal conductivity and thermal capacitance. The first is the ability of the material to conduct heat (or cold), the second is the ability of the material to hold on to that heat (or cold).
When it comes to tempering you need a material that will conduct heat well enough to cool down fairly quickly, so that your slab will be cool in a cool room or easily cooled with refrigeration. That's the conductivity part. But at the same time you want that material to hold on to that cool temp for an extended period of time while you place warm chocolate on it. That's the capacitance part. That part is actually even more important.
This is why people don't use metal to temper chocolate. Its conductivity is terrific. Better than marble. But its capacitance is crap. In other words metal heats up or cools down easily. But it loses that heat or cool too fast for this purpose.Marble is used because it hits the sweet spot of cooling relatively quickly, but holding onto that cool for a longer period of time. In other words just the right mix of thermal conductivity and thermal capacitance for this job.
Can you get other stone or artificial materials to work for you? Probably, but you'll want to check their thermal properties compared to marble first. Tempering chocolate with this method is finicky to begin with, and there's no reason to make your job harder than it has to be.
I wouldn't overthink it to be honest. if you're tempering on a slab, you're using small quantities of chocolate. I've never done the math, but suffice it to say that i temper more than the average bear, and i've never encountered the thermal limit of the stone whereby it had absorbed so much heat that it no longer was an effective heat sink for my chocolate. Mike's right in his answer, but the chances of you tempering so much that you saturate your stone is very low. If you are tempering so much chocolate that you hit your stone's thermal limits, you're at the point where your equipment is jacketed and made of stainless steel, not rocks.
Thank you Michael and Sebastian, very helpful information. I should also mention that I'm actually tempering fudge, but I believe the points you both made still hold true for my purposes. I've only been using a marble slab, but getting ready to expand my work area and thought I'd consider other stones surfaces. But I must confess, I do like the elegance of the carrera marble slab.
Thanks again gentlemen!