Mustard seed (Brassica sinapis): The seed of the various kind of Brassica differ only slightly in size. In color they range from the pale yellow of the White mustard seed, Sinapis alba, to the dark reddish brown of the Black mustard seed, Brassica nigra. The tiny seed of the white variety is from 3/64 to 3/32 inch in diameter; and some patient soul counted more than 5,000 of these tiny little globes in a single ounce. The seed is round and clean, with tiny pit marks showing on the surface when it is seen through a microscope. There is no aroma whatsoever to the whole seed. However, when it is crushed and the resulting powder or flour is mixed with water, its taste is at once pungent, sharp, and piercing. When one smells the pure, newly blended paste, the result is often so irritating that tears come to the eyes. The black mustard seed is of the same appearance as the white but is considerably smaller. It takes more than 12,000 of these tiny seeds to make an ounce. The flour from this crushed seed is more pungent than that of the white. California and Montana produce good crops of mustard seed but not enough to supply demand; the finest grade of seed still comes from England and Holland. Other than being crushed into flour for use in prepared mustards and condiments, the whole seed are useful in pickling and preserving. Many popular pickle recipes call for mustard seed, especially those for sweet mustard pickles.

Source: Miloradovich, Milo (1950). Cooking with Herbs and Spices. New York: Dover; pp. 119-120.