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.
In roman times, this rosy-pink stone was symbolic of love and reconciliation, and was often given as a gift of love. The pink tones of naturally occurring rose quartz are caused by iron and titanium. However, most rose quartz in the bead market is dyed to maintain a uniform color. Both natural and dyed rose quartz will fade in bright sunlight, so take care to avoid over exposure.