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 anniversary for the 50th anniversary, and every fifth anniversary after that. In the United States, a couple can receive a greeting from the President for any wedding anniversary on or after the 50th. Roman Catholics may apply for a Papal blessing through their local diocese for wedding anniversaries of a special nature (25th, 50th, 60th, etc.). The names of some anniversaries provide guidance for appropriate or traditional gifts for the spouses to give each other; if there is a party these can be brought by the guests or influence the theme or decoration. These gifts vary in different countries, but some