Key tops were generally made of elephant ivory.  A few of the piano
makers and keytop makers (like Pratt Reed) had photos of them stacked
in their storerooms, and they are obviously not walrus.  Some keytops
were made of walrus, also, but I doubt very many.  I'd be interested
to know.  I'm sure one could tell the difference under a microscope.

Today, re-ivoried keys can be done in mastodon tusk, which is available
for sale (no danger of extinction there!), or cow bones.  The latter
has larger pores than ivory, but looks pretty good.

Philip Jamison
Pennsylvania