Dark opal basil is a cultivar of Ocimum basilicum (sweet basil), developed by John Scarchuk and Joseph Lent at the University of Connecticut[2] 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.

'Dark opal', along with other large leaved purple cultivars such as 'Purple Ruffles', has a high concentration of anthocyanins and is considered a potential source of red pigments for the food industry.

The anthocyanin concentration is about 18 mg per 100 g fresh leaves, similar to Perilla frutescens, and greater than smaller leaved purple basils such as 'Purple bush' (at about 6 mg per 100 g).