Blue Banded Onyx, a Chalcedony, closely related to the Quartz family, though it is not a genuine “crystal”, as Quartz is, rather is it cryptocrystalline, looking like but not quite it.
Along the border area of Spain and Portugal is found a variety of banded Onyx in blues and purples and greys. The particular brightness in some of the blue of this pair shown above invited a query as to the pair being natural or treated, and the supplier gave assurance that the colour of the banding is natural, not treated.