Rai are the small round seeds of various mustard plants. The seeds can come from three different plants: black mustard (Brassica nigra), brown Indian mustard (B.juncea), and white mustard (B.hirta/Sinapis alba). The seeds are usually about 1 or 2 mm in diameter. Mustard Seeds may be colored from yellowish white to black. They are important spices in many regional foods.