Strontium is a soft, silver-yellow, alkaline-earth metal, existing either in the elemental state or as the divalent ion. Strontium occurs naturally in the earth's mantle as a mixture of four stable isotopes, i.e. Sr-84, Sr-86, Sr-87, and Sr-88. Of these, Sr-88 is the most prevalent form, comprising about 83% of natural strontium. Natural stable strontium is abundant in the environment and can be found in all plant species. Consuming 75% of production, the primary use for strontium is in glass for color television cathode ray tubes, where it prevents X-ray emission. Acantharea, a relatively large group of marine radiolarian protozoa, produce intricate mineral skeletons composed of strontium sulfate. In biological systems, calcium is substituted in a small extent by strontium. In the human body, most of the absorbed strontium is deposited in the bones. The ratio of strontium to calcium in human bones is between 1:1000 and 1:2000, roughly in the same range as in the blood serum. The human body absorbs strontium as if it were its lighter congener calcium. Because the elements are chemically very similar, stable strontium isotopes do not pose a significant health threat. The average human has an intake of about two milligrams of strontium a day.