solved...

Mystic quartz (and mystic topaz, aura or aurora borealis) are natural gemstones that have a coating of titanium applied by CVD (chemical vapour deposition).

Faceted mystic quartz and mystic topaz have the coating applied to the bottom or pavilion facets of the stone. Once the stones are set in jewellery this coating has some protection from the setting but care should be taken not to damage it prior to then.