Without seeing it, I'd guess possibly jet spinel, an often faceted and
expensive gem, or onyx, softer, more common and smoothed surfaces.
Google both for images.
Another less pretty name is bloodstone, a form of jasper that often
bumps up next to green. Gemologists identify it as this. Searching for
"Bloodstone" online often shows a translucent, all-red stone, not at all
the same thing! Check this out:
https:...
