I also think its "Onyx" (Aragonite, I believe) either from south america or Pakistan. The kind made into ashtrays, sculptures and coffee-table tops!
The hole wouldnt be that large and regular if it was agate.
It could have been turned to make it such a regular shape.
If it is onyx, it will scratch with a penknife or a steel pin.
Was it found in East Africa?
Stefany