ABC Wednesday has reached the difficult letter X.
After long thought (and some research) I've decided to offer
The name xenolith comes from the Greek for "foreign rock". In strict geological terms it's a rock which has become embedded in a different type of rock while the latter was in a molten state (see this article) although the definition can be stretched to include a foreign fragment included in a sedimentary rock.
So I'm entitled to stretch a bit!
The series of photos below show a rough piece of slate (sedimentary rock) enclosing an almost spherical group of bright pyrite crystals.