Okay, maybe I'm dumb, but what's the difference between these two types of calling surfaces? Is it the way they are produced? they both seem like glass to me...

Yeah you're on the right track."Glass" is our regular stuff which comes from silica,that is found in sand.While "crystal glass"has rock crystal added in it.It makes it harder,and is generally more high pitched.They do look the same.You may have to cut deeper grooves into crystal to get it to sound better,as sandpaper smooths it out,and it needs to be a little rough to make a good sound without getting those high pitched squeaks we don't want to make.If I'm wrong help me out guys...

You will need to use rougher sand paper on crystal,I use 60 grit to get the deep scratches,then conditon it as needed.

Good to know stuff, gonna put that in my little note book: let's see,
glass vs. crystal! THanks!

You should condition both of these calls with a Stone instead of paper , you will notice a big difference in perfomance and Tone...BT