After Burma, Siam (now Thailand) is the second most well known source of fine rubies.

It is unclear when ruby mining in Siam started, but a Chinese traveller in the early 15th century made one of the earliest references to Siamese rubies.
One of the first European explorers, Fernand Mendez Pinto, who visited Siam in 1548, noted: "In the Kingdom of Siam are found rubies and sapphires in abundance."
The Siam's ruby deposits are found exclusively in the adjoining provinces of Chantaburi and Trat, which lie some 300-km southeast of Bangkok, not far from the Cambodian border.

The cutting and trading centre for rubies in Thailand was and still is Chanthaburi, a busy provincial town where not only rubies but also sapphires and others stones are sold.
Today, of course, all gems find their way to Bangkok, the capital of the country and second trading- centre.

Compared to Burma rubies, Siam rubies tend to be of a darker shade, often with brown undertones and, due to the presence of iron along with the chromium oxide approach in general more the burgundy red. However, very fine Siam rubies can nearly rival fine Burmese stones in quality. The big majority of all Siam rubies are heat-treated.