With the recent drastic decline in production from the classic ruby mines of Burma, Thailand has become the world's major source of gem ruby. The majority of the ruby production, as well as some significant sapphire production, comes from the provinces of Chanthaburi and Trat in southeastern Thailand. This article reviews the geology of these alluvial deposits and describes the wide range of methods used to mine the corundum. Gemologically, the Thai rubies are distinguished by their violet overtone, inhibited fluorescence, and characteristic inclusions. Also discussed is the heat treatment of corundum to enhance color, which has become a major industry in Chanthaburi. The newest product, "golden" sapphire, is examined, including the apparent stability of the color produced and clues to the detection of the heat treatment.