Sardonyx is the same stone as onyx, a banded agate with regular and uniform stripes. Onyx has a black base, sardonyx has a brown base. Like most agates, sardonyx and onyx are often dyed for color enhancement and uniformity.
1 ST $16.73 ($16.73 per ST) was $23.90
2 ST $23.91 ($11.95 per ST) was $34.15
3 ST $31.36 ($10.45 per ST) was $44.80