I tend to favor the Woodhaven brand of pot and peg calls, because of their excellent quality and tone.

One thing about slate...if it gets wet or is damp outside, the tone quality tends to degrade significantly. If this is to be the only friction call in your bag, I would definitely look at one of the other surfaces, such as glass, crystal or aluminum. They tend to hold their tone in all weather.

With slate:

If it's black, put it back..if it's gray, let it play.