Dark opal basil is a cultivar of Ocimum basilicum (sweet basil), developed at the University of Connecticut in the 1950s. With deep purple, sometimes mottled leaves, it is grown as much for its decorative appeal as for its culinary value. Dark opal basil was a 1962 winner of the All-American Selection award.
Like other purple basils, the purple color is from anthocyanins, especially cyanidin-3-(di-p-coumarylglucoside)-5-glucoside but also other cyanidin based and peonidin based compounds. 'Dark opal', along with other large leaved purple cultivars such as Purple ruffles basil, has a high concentration of anthocyanins and is considered a potential source of red pigments for the food industry. The anthocyanin concentration is about 18mg per 100g fresh leaves, similar to Perilla frutescens, and greater than smaller leaved purple basils such as Purple bush basil (at about 6 mg per 100 g).