There are no evident samples on display at Petra, the Stone Atelier, a stone showroom and supplier in Valencia, Spain. Instead, sales associates lead clients to a luminous area at the rear of the minimalist space, where glazed offices and conference rooms on each side flank a 7-foot-high circular wall at its center. But as a segment of this ringlike form pivots open, visitors will discover a secret room lined with a contiguous series of discreet flush doors, each opening to reveal a slab of one of 36 varieties of stone, all organized chromatically from light to dark and evenly illuminated from above. The project’s architect, Valencia-based Fran Silvestre, rejected more conventional display systems such as racks with sliding panels as too much “like a bazaar.” Summing up the client’s upscale marketing strategy, he explains, “The better the product, the less of it you should see on display.”