What kind of rock or crystal is mostly black but looks like it has oil slick spots on it?

The rock or crystal you are describing is likely labradorite, a type of feldspar mineral. It typically appears dark, often black or gray, but exhibits a phenomenon called labradorescence, which creates iridescent spots that resemble oil slicks in various colors. This striking visual effect makes labradorite a popular choice for jewelry and decorative items.