Agate (Orca)

Madagascar-origin material Orca Agate is a striking banded form of chalcedony (microcrystalline quartz, SiO₂), part of the quartz / chalcedony group, featuring alternating black and white patterns reminiscent of an orca whale. It forms in volcanic rocks, particularly basaltic lava flows, where silica-rich fluids filled cavities over time. Chemical variations in these fluids caused the characteristic bands: dark layers are quartz-rich, while the softer white zones are often calcite (CaCO₃) inclusions, which can be scratched with a fingernail. The Madagascar deposits, particularly in southern and central volcanic fields, produce some of the most dramatic and high-contrast specimens. Orca Agate from Madagascar forms in gas vugs and cavities in basalt, where repeated deposition of silica produced layered chalcedony bands.

During these periods, darker bands incorporated iron or manganese oxides, producing black or gray colors. White bands, however, often formed under conditions where calcium carbonate (calcite) precipitated instead of silica, resulting in softer, chalky layers. This combination produces the stone’s iconic black-and-white “orca” pattern. Specimens are typically cut into slabs, cabochons, spheres, and decorative pieces, showcasing the contrast between hard quartz bands and softer white calcite patches.