|Diamond Alkali, Shamrock Oil and Gas, and Sigmor|
|Industry||Oil, gas, chemicals|
|Founders||John Sheerin, TR Evans, Thomas E. Turner|
|Headquarters||San Antonio, Texas|
Diamond Shamrock Corp. or Diamond Shamrock Refining and Marketing was an oil refinery and gas station company in the United States, headquartered in San Antonio, Texas.
The origins of Diamond Shamrock can be traced back to three foundation companies: Diamond Alkali, Shamrock Oil and Gas, and Sigmor Corporation.
In 1910 a group of glass manufacturers founded Diamond Alkali in Pittsburgh, Pennsylvania. They wanted the company to produce soda ash, a key ingredient in glass production. A factory was built in Painesville, Ohio, in 1912 to produce soda ash. During the 1920s, TR Evans led Diamond Alkali, which under his leadership became an important chemical producer. After World War II Ray Evans, TR's son, led the company to decentralize its operations. In Deer Park, Houston, Texas, in 1946 a new plant was built to produce Chlorine and Caustic soda. In 1948 the company moved its headquarters from Pittsburgh to Cleveland, Ohio. During the 1950s a third plant was constructed in Muscle Shoals, Alabama, helping the company continue to enlarge its range of products, expanding to produce plastics and chemicals for agriculture. The 1960s saw continued expansion of Diamond Alkali. A facility was opened in Delaware City, Delaware and additional chemical companies were purchased, including Chemical Process Company of Redwood City, California and the Nopco Chemical Company of New Jersey. In 1967 Diamond Alkali merged with Shamrock Oil and Gas of Amarillo. At the time of the merger the company produced about 20 percent oil and gas and 80 percent chemicals. In 1978 Diamond Shamrock moved its headquarters to Dallas. By 1980 Diamond Shamrock had about 12,400 employees in thirty-seven countries.
Shamrock Oil and Gas
John Sheerin founded Shamrock Oil and Gas on August 9, 1929. As a native of Ireland he named the company after the symbol of his country of origin. The company was financed by the Fownes family of Pennsylvania and headquartered in Amarillo. The early years of the company were difficult experiencing a loss of about $9 million. In 1933 Shamrock built its first refinery and its first gas station, both in Sunray, Moore County, Texas. James Harold Dunn joined the company in 1938 as a vice president and general manager, having previously been an engineer at the Lone Star Gas Corporation. The following year Shamrock showed its first, albeit small, profit. During 1939-1940 Lone Star and Shamrock cooperated on the construction of a plat at Murchison in Henderson County whose aim was to recycle natural gas. In 1943 the company paid its first dividend, and by 1944 the company was listed on the NYSE. In 1945 Dunn became the company president, a post he had for ten years. In 1955 CA Cash succeeded him, and in 1959 Shamrock opened its first catalytic cracking unit in Sunray.
In 1960 Shamrock purchased a large number of gas stations from the chain of Sigmor. 
During the 1930s and 1940s Sigfried (Sig) Moore operated the chain. In 1943 Moore loaned Thomas E. Turner, an employee, money to launch his own business. Turner decided to use the name Sigmor for the chain of stores he established during the 40s and 50s. In 1952 Sigmor was incorporated. In 1959 a restructuring took place which allowed each separate gas station to incorporate separately. In 1960 most of the chain was purchased by Shamrock and then leased back to Turner, who continued to lead the company. In 1978 Sigmor purchased its stations back from Diamond Shamrock, continuing to market DS products. By 1983 Sigmor was one of the largest independent service-station chains in the USA. The merger gave Diamond Shamrock 600 retail outlets, plus the Three Rivers oil refinery which was built by Sigmor during the 1970s. In 1987 The Diamond Shamrock Refining and Marketing Company severed ties with Diamond Shamrock Corporation, which was the parent company, and became independent with its headquarters in San Antonio. At that time, as part of the reorganization, Diamond Shamrock Corporation became Maxus Energy Corporation, severing all legal ties to the Diamond Shamrock Refining and Marketing Company. In 1990 Diamond Shamrock Refining and Marketing Company shortened its name to Diamond Shamrock, Incorporated, as it had taken the step of incorporating at that time. 
In 1985 the company had a $604.7 million loss. It restructured itself in the year before December 1986. On Wednesday December 3, 1986 T. Boone Pickens offered to buy Diamond Shamrock for $2 billion. Kit Freiden of the Associated Press stated "some analysts predicted the giant energy company would reject the offer."
In 1988 the company's annual refinery sales were $1.8 billion. Diamond Shamrock owned and operated 529 stores. 423 of them were in Texas, with 94 of them in Greater Houston. The company owned an additional 64 stores in Colorado. In 1988 the company bought from investor F. Philip Handy 80 Tenneco gasoline stations, with 30 of them in Houston. According to the plans, the Tenneco stores would be rebranded as Diamond Shamrocks.
In 1995 Diamond Shamrock had 2,000 stores, with most of them in Texas, Colorado, New Mexico, and Louisiana. Of them, over 170 stores were in Houston. That year, Diamond Shamrock bought the National Convenience Stores Stop N Go chain for $260 million. The plans called for the combined company to be headquartered in San Antonio. The combined company was to have two refineries in Texas, 11,000 employees, and 2,600 stores.
- Odintz, Mark (12 June 2010). "Diamond Shamrock". Handbook of Texas Online. Texas State Historical Association. Retrieved 3 September 2015.
- Odintz, Mark (12 June 2010). "DIAMOND SHAMROCK". Handbook of Texas Online. Texas State Historical Association. Retrieved 3 September 2015.
- Freiden, Kit. "Diamond Shamrock may reject Mesa bid." Associated Press at the Houston Chronicle. Thursday December 4, 1986. Business p. 2. Available at NewsBank, Record Number HSC1204425523. Available at the Houston Public Library website with a library card.
- Pearson, Anne. "Diamond Shamrock to buy gas stations." Houston Chronicle. Tuesday December 6, 1988. Business p. 3. Available at NewsBank, Record Number 12*06*588337. Available at the Houston Public Library website with a library card.
- Hassell, Greg (9 November 1995). "Diamond Shamrock buys Stop N Go chain". Houston Chronicle. p. Business 1. Archived from the original on April 13, 2013. - Available at NewsBank, Record Number HSC11091306995. Available at the Houston Public Library website with a library card.
- "Ultramar to Buy Gas Retailer for $1.96 Billion". Los Angeles Times. Bloomberg Business News. September 24, 1996. Retrieved May 12, 2014.
- "Company Overview of Ultramar Diamond Shamrock Corporation". Bloomberg Businessweek. Retrieved May 12, 2014.
- "Valero Energy Corporation and Ultramar Diamond Shamrock Corporation". U.S. Federal Trade Commission. May 14, 2002. Retrieved May 12, 2014.