Looks like a bit of well rounded coal.
Best test is to knock a small piece off and stick it in the charcoal of a BBQ. If it smokes it is probably coal. If it bubbles & melts it is asphalt.
Should also weigh less than most other rocks.
Occasional other similar items are black iron rich glass (occasional in patio, & upmarket bath hot tub tiles),or obsidian (occasional in coastal landfill but usually close to source.
Other similar items are... Pitchblende (very heavy), Jet (a type of fossilized wood) & also combustible; Tourmaline (usually black but tends to be relatively small, occasional gem quality tourmaline borosilicates are multi coloured rather than black as a result of the loss of the usual Fe3+ & Fe2+ mixed black/brown stain). large rounded black crystals are results of erosion of a pegmatite (very rare), Tourmaline will weigh much the same as a classical rock. Tourmaline often forms slightly linear black bits in coarse sandstones derived from granite source rocks, such as the Cretaceous Greensands of S. England with one sediment source there being the Cornubian batholith (Dartmoor, Bodmin Moor, Lands end, Scilly isles etc.
May also on very rare occasions have black flint but this is usually a dark grey due to inclusions of chalk dust.
Have a nice day: Ag