This is an extraordinary Sidi Rahal specimen built around a large blocky chalcedony form that showcases the geometry of an earlier cubic mineral. The exact precursor remains debated in the collector literature. Some Sidi Rahal specimens are documented as quartz after fluorite, including pieces where the cubic form and later generations of quartz growth are clearly preserved, while other examples have been sold as chalcedony casts after halite. 

The main side is wildly textural. Successive generations of chalcedony have developed across the cubic form in swirling, almost concentric bands, with a pronounced central eye rising above the surrounding growth. That secondary chalcedony growth gives the specimen a character quite unlike the cleaner cubic pseudomorphs from the same area. Even amongst Sidi Rahal material, the few examples I have tracked down vary enormously in form, and this feels like its own very individual manifestation of that same mineralising process.

There is beautiful crystallisation scattered across the main side as well, with soft cream, pink and iron rich colouring catching the light far better in person than the camera manages to show. On the reverse, the piece changes character completely, with lustrous ferruginous quartz adding surprise, sparkle, warmer colour and another generation of growth.

I've been trying to find this kind of material for a long time, ever since seeing the strange cubic Sidi Rahal pseudomorphs and casts that occasionally surface from old collections. This is a substantial example, and the amount of detail packed into the chalcedony growth is phenomenal. The square form gives away the earlier cubic mineral, but the later swirling growth, central eye and ferruginous quartz make this piece far more than a straightforward pseudomorph.