text: <unk>  is the hardest and most highly mineralized substance of the body, and with dentin, cementum, and dental pulp is one of the four major tissues which make up the<unk> . It is the normally visible dental tissue of a<unk>  and must be supported by underlying dentin. Ninety-six percent of<unk>  consists of mineral, with water and organic material composing the rest. The normal color of<unk>  varies from light yellow to grayish white. At the edges of teeth where there is no dentin underlying the<unk> , the color sometimes has a slightly blue tone. Since<unk>  is semitranslucent, the color of dentin and any restorative dental material underneath the<unk>  strongly affects the appearance of a<unk>  varies in thickness over the surface of the<unk>  and is often thickest at the cusp, up to 2.5 mm, and thinnest at its border, which is seen clinically as the cementoenamel junction (CEJ).<unk> 's primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. The large amount of minerals in<unk>  accounts not only for its strength but also for its brittleness.<unk>  is the hardest substance in the human body, ranking a 5 on Mohs hardness scale. Dentin, less mineralized and less brittle, 3-4 in hardness, compensates for<unk>  and is necessary as a support.
  Unlike dentin and bone,<unk>  does not contain collagen. Instead, it has two unique classes of proteins called amelogenins and<unk> ins. While the role of these proteins is not fully understood, it is believed that they aid in the development of<unk>  by serving as a framework support, among other functions.
option_0: Tooth enamel
option_1: Skin appendage
option_2: Tissue 
option_3: Human skeleton
option_4: Hair
option_5: Bone
option_6: Cartilage
option_7: Olea