Mustard Seeds. Mustard seeds of the various mustard plants are among the smallest of seeds. The seeds are about 3mm in diameter, and may be colored from yellowish white to black. They are important spices in many regional cuisines. The seeds can come from three different plants: black mustard (Brassica nigra), brown Indian mustard (B. juncea), and white mustard. Indians have used mustard seeds in their food for over two thousand years. Mustard seeds are also known as "Sarson" and is very popular in North India. In North sarson ka Saag is very popular. "Sarson ka tel" Mustard oil is very good for body massage during extreme winters, as it keeps the body warm and moist.