To me it looks like chert or *Chalcedony, I must admit that I am at a loss to make a mineral specific ID from the photos. However I am quite certain it is chalcedony, because of the look of waxy smoothness, color, and the patina.
Pure black chalcedony is fairly rare. Onyx is black chalcedony with straight thin lines or stripes of black and white. Although pure black chalcedony is commonly referred to as Onyx in error.
"However the "look" of a mineral is no sure ID, only tests can ID it properly.

Everything else is just a guess." -Eu_Citzen-
* Chalcedony is a variety of the mineral quartz that occurs in in a great variety of colors including blue, black, lavender, white, buff, light tan, gray, yellow, pink, red or brown. If chalcedony is conspicuously color-banded, it may be called agate and with other minerals it has various other names including chert, flint, jasper, carnelian, mocha stone, onyx, chrysoprase, heliotrope, and bloodstone to name a few.
Although with that said, Obsidian is still a possibility.
GG~