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
Anniversary to as the "traditional" list. Generally speaking, the longer the period, the more precious or durable the material associated with it. See wedding anniversary for a general list of the wedding anniversary symbols; however, there are variations in some national traditions. Furthermore, there exist numerous partially overlapping, partially contradictory lists of anniversary gifts (such as wedding stones), separate from the 'traditional' names. The concepts of a person's birthday stone and zodiac stone, by contrast, are fixed for life according to the day of the week, month, or astrological sign corresponding to the recipient's birthday. Anniversary An anniversary is the date