I found that one very interesting and it just so happened that the mountain the castle was built on consisted partly of slate. So I pocketed a portion of said mountain for later use and here is the first result.It was not difficult at all to decide which pony should be the first to be engraved in “fine-grained, foliated, homogeneous metamorphic derived from a shale-type sedimentary rock compsed of clay or volcanic ash through low-grade regional metamorphism” as Maud might call slate without any need to look it up on Wikipedia ;-)
I’m happy with the result given that it is a first try. Slate turned out to be a rather different material to work with than soapstone or glass.