The history Diamond Shamrock can be traced back to the Diamond Alkali company and the Shamrock Oil and Gas company.
Diamond Alkali was begun in Pittsburgh, Pennsylvania in 1910 by a group of glass manufacturers. By the 1920s, it was a successful chemical company that produced cement, calcium carbonates, coke, chlorine, and more. By the 1950s, the company extended into the plastics industry and the agricultural chemical industry and in 1967, it merged with Shamrock Oil and Gas.
Shamrock Oil and Gas was founded in 1929 in Amarillo, Texas by an Irishman named John Sheerin. In 1933, the company built its first refinery and opened its first gas station. By 1941, there were over 160 Shamrock gas stations and in 1967, the company merged with Diamond Alkali to form Diamond Shamrock Corporation.
In 1983, Diamond Shamrock merged with Sigmor Corporation. At the time, Sigmor was one of the largest independent service station chains in the country, so when the two companies merged, Diamond Shamrock acquired around 600 retail outlets. In 2001, Diamond Shamrock was acquired by Valero and although customers can’t apply for a Diamond Shamrock gas card, they can apply for a Valero gas card.