What is a Geuda stone?

A geuda is a naturally occurring, heavily included, and milky form of pale corundum found in Sri Lanka that looks visually worthless but can turn into a vivid blue sapphire once it’s properly heat-treated. It forms a huge part of the island’s gem output.

Long ago, geuda stones were thrown away by miners or treated as gravel and nothing more. In the 1970s, Thai gemmologists noticed that using extreme, controlled heat dissolves the internal rutile silk, and this improves transparency in a big way, leading to deep blue tones. These days, the geuda trade is a major industry, and it helps feed the commercial sapphire market around the world.