Calcium exists in large quantities in the human body, constituting the skeleton as a scaffolding to support the body, and also plays an important role in secretion, transportation, muscle contraction, and nerve conduction. Calcium exists in two forms in the body, one is ionized calcium, which exists in the blood circulation, so-called blood calcium; the other is bound calcium, which combines with protein, carbonic acid or phosphoric acid and is deposited in tissues, except for bones and teeth, when calcium permeates all tissues and cells, and does not appear in a solid state in tissues in general; however, in some cases, calcium precipitates into a solid and is deposited in tissues, which is a pathologic calcium salt deposition. The calcium salts deposited are primarily calcium phosphate and to a lesser extent calcium carbonate. Calcium salts are usually monorefractive, but calcium oxalate is birefractive, and when stained with HE, calcium generally appears as a purplish-blue color, and a number of dyes can form chelates with calcium, including alizarin red S, red violet, and nuclear solid red.

Alizarin Red S is an anthraquinone derivative, a sodium salt of alizarin sulfonate, which chelates with calcium salts in calcium carbonate or calcium phosphate to form an orange-red complex. Alizarin Red S tends to stain small amounts of sediment to give more reliable results, and is often used in conjunction with solid green or Mayer's Hematoxylin stains to combine to form an orange-red precipitate, which is suitable for staining small quantities of calcium salts in tissue.