Silica, or silicon dioxide, can occur in a crystalline or noncrystalline (amorphous) form. Below this temperature, honey enters a glassy state and becomes an amorphous solid (noncrystalline). Sugar candies can be classified into noncrystalline and crystalline types. Glass is often not considered a ceramic because of its amorphous (noncrystalline) character. Polymers display varying degrees of crystallinity, and many are completely noncrystalline. Viscoelasticity is a form of nonlinear elasticity that is common amongst noncrystalline polymers. Noncrystalline ceramics, being glass, tend to be formed from melts. In addition, the same atoms may be able to form noncrystalline phases. Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A", have been proposed.