The blue pigment created by ancient Egyptians from copper-bearing materials and sand was Egyptian blue. Chemically, its principal blue phase is calcium copper tetrasilicate, also called cuprorivaite, with the formula CaCuSi₄O₁₀.
It was not simply ground-up blue stone. Craftspeople heated silica from sand, a copper compound, calcium-rich material and an alkali such as natron or plant ash. The resulting solid was ground into powder for paint or shaped into objects such as beads, inlays and faience-related materials.
Egyptian blue is often described as the world’s first synthetic pigment. Evidence for its use reaches back to Egypt’s Fourth Dynasty, and it remained important through the Middle Kingdom, New Kingdom and Greco-Roman periods. It appeared on stone, plaster, wood, papyrus and sculpture.
Several alternatives in the quiz are natural or later pigments. Azurite is a naturally occurring copper mineral, lapis lazuli is a semiprecious stone, and cobalt blue is a much later synthetic pigment. Egyptian blue’s recipe was eventually forgotten after late antiquity, though modern researchers have reconstructed it.