Ruby is the red variety of corundum that is coloured through chromium. Ruby forms in metamorphic (e.g. Burmese marble and Mozambique amphibolite) deposits, or magmatic geological environments. Untreated high-quality ruby is one of the most expensive gems on the market. In May 2015, Sotheby's achieved a new world record price for a ruby at auction by offering the 25.59 ct 'Sunrise Ruby' ring that was purchased for US$ 30 million. Ruby is often treated to enhance its colour and/or transparency. Low-quality rough samples may be treated with lead glass, a treatment that has become widespread in the past decade. Ruby mines today are found in Burma, Mozambique, Vietnam, Afghanistan, Madagascar, Tajikistan, Tanzania and Kenya.