It's true: the flour used for the corn tortillas so integral to Mexican food, masa harina, really does have ground-up stone in it -- but don't freak out about it. The limestone used in the mix is very fine, not gritty at all, and helps to hold together the thick masa tortilla dough, which is otherwise made of ground corn kernels.
Always, always wear thin plastic gloves or plastic bags on your hands when handling any kind of hot peppers while preparing your Mexican food. This makes it easy to avoid getting the oils on your hands. Even more important, don't ever touch your face or eyes while working with them; the result can be very painful.