A New Discovery of Common Opal from Antetezambato, North-western Madagascar

In 2019, a yellow-green variety of common opal was discovered at Antetezambato in north-western Madagascar, within a well-known deposit of demantoid garnet. This article describes the unusual geological context of this opal occurrence and provides a detailed characterisation of this new gem material using a multidisciplinary approach. The material consists of microcrystalline opal-CT, and contains a greater fraction of tridymite than cristobalite. Chalcedony fills microscopic pores and voids within the opal. The yellow-green colour of this gem material is primarily attributed to iron, which is present mostly in mineral inclusions (i.e. epidote and iron-bearing minerals such as goethite). The distribution and interplay of these inclusions contribute to the opal’s distinctive appearance and natural variations in colour intensity. Due to its durability and aesthetic qualities, opal from Antetezambato is suitable for use in ornamental applications and jewellery, including cabochons, spheres, carvings and mosaics