A flinty, black mineral, in ancient times it was known as the touchstone.
A flinty, black mineral, in ancient times it was known as the touchstone. If pure gold were scratched on it, it would leave yellow marks, while gold mixed with copper would leave red marks, thus giving a way of assessing the purity of gold. A metamorphic rock, it is composed of laminated, often flaky parallel layers of chiefly micaceous minerals. FTP, identify this rock, whose name comes from the Latin for "split", and sounds like a bad word.
A flinty, black mineral, in ancient times it was known as the touchstone.
Schist