I suppose you would call slate stone. After all it is derived from shale type sedimentary rock 400 million years or more in the making. I am reminded of the ads for Bulmer’s cider here with its slogan “Time dedicated to you”.
So when you acquire a Heartworks piece, you are “Buying Time” even if it isn’t a clock you are purchasing. “Time etched in stone” is what you get. And, as slate has been used for centuries as roofing material and thus is an intrinsic part of our Irish heritage, you get that too.