I am retelling the exact story Yousef Awyan told me in the Cairo Museum about the Schist Disk!! I'd never heard of this backstory about it's possible use before!! If it turns out to be correct then ...

The only exception to the rule is Country Tyrone which has a bit of everything - clay, basalt, gabbro, granite, schist ... to form a new type of rock called sedimentary rock.

There are three main types of rock — igneous, sedimentary and metamorphic ... with examples including phyllite, schist, quartzite and marble. Igneous and metamorphic rocks on Earth's surface ...