the finish is several coats of polyurethane.

left pic shows walnut over padauk, right pic shows hard maple (backed by walnut) over obeche

from the left: hard maple (shown front-on in the top right pic), walnut, goncalo alves, padauk, walnut, hard maple veneer, purpleheart, osage orange, walnut, obeche, walnut, unknown, and walnut.