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 $20.32 ($20.32 per ST) was $23.90
2 ST $29.03 ($14.51 per ST) was $34.15
3 ST $38.08 ($12.69 per ST) was $44.80