Pennswoods makes a really great slate call...Here ya go..
Premium Slate Call
A high quality wood pot call featuring (2) different striking surfaces - slate and glass. The top surface features fine, thin, Portuguese slate for making sweet sounding yelps and cutting. The underside glass surface is designed to make sweet sounding purrs and clucks when that gobbler is within 75 yards. Comes with hickory and waterproof strikers. http://www.pennswoods.com/9902.html