Jet-black stone

ONYX

This glossy gem is often used in cameos and was historically believed to protect against negative energy in ancient cultures.

"Jet-black stone" is a straightforward definition clue pointing to onyx, a smooth, deep black gemstone often used in jewelry. The word "jet" reinforces the color reference—jet itself is another black stone, so the clue subtly hints at the answer by describing its most striking feature. No wordplay here, just a clear description matching onyx’s well-known appearance.