Crystal structure "white tin" respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several GPa. White tin is metallic, and is the stable crystalline form at or above room temperature. Below 13.2 °C, tin exists in the gray form, which has a diamond cubic crystal structure, similar to diamond, silicon or germanium. Gray tin has no metallic properties at all, is a dull gray powdery material, and has few uses, other than a few specialized semiconductor applications. Although the α–β transformation temperature of tin is nominally 13.2 °C, impurities (e.g. Al, Zn, etc.) lower the
Wedding ring phrase of significance to the spouses. In many nations the engagement rings are plain while the bride's wedding ring commonly is bejeweled. Some customs include the wedding ring as the final of a series of gifts, which also may include the engagement ring, traditionally given as a betrothal present. This custom was practiced in ancient Rome and is possibly much older. In several traditions, the best man or maid of honour has the duty of keeping track of a couple's wedding rings and to produce them at the symbolic moment of the giving and receiving of the rings during the