Mustard seeds are the small round seeds of various mustard plants. The seeds are usually about 1 to 2 millimetres in diameter and may be colored from yellowish white to black.Also called ‘black mustard seeds’ these are added to pickles and curries. Mustard is an annual herb which has been used since the earliest recorded history. An ingredient in the Indian spice blend ‘panch phora’ and used in many dishes for flavour and appearance. Black mustard (Brassica nigra) is almost identical to brown mustard and one is commonly substituted for the other. Yellow mustard (Brassica alba) is a larger seed and generally has less heat.