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 customs by country were considered a personal event and inviting people to the wedding who are not known to at least one member of the couple well enough to be able to choose an appropriate gift was considered inappropriate, and registries should therefore be unnecessary. Whether considered appropriate or not, others believe that weddings are opportunities to extract funds or specific gifts from as many people as possible, and that even an invitation carries an expectation of monetary reward rather than merely congratulations. Letters of thanks for any gift are traditionally sent promptly after the gift's receipt. Tradition allows wedding gifts to be