There are indications of sapphires and other forms of corundum.
The minerals of Siam include gold, silver, rubies, sapphires, tin, copper, iron, zinc and coal.
Under artificial illumination many sapphires appear dark and inky, whilst in some cases the blue changes to a violet, so that the sapphire seems to be transformed to an amethyst.
Some of the slightly cloudy Ceylon sapphires, usually of greyish-blue colour, display when cut with a convex face a chatoyant luminosity, sometimes forming a luminous star of six rays, whence they are called "star sapphires".
In 1875 sapphires were discovered in deposits of clay and sand in Battambang (Siam), where they have been worked on a considerable scale.