If the characteristics referred to in the title were their only quality, fulgurites would be fascinating. However, they have other unusual qualities that make them even more amazing. For example, some hold ancient air within that can offer a window into palaeo-environments. Fulgurites are natural tubes or, in the case of rock fulgurites, crusts of glass formed by the fusion of silica (quartz) from a lightning strike. They are categorised in four main types: clay, sand, caliche and rock fulgurites. In the case of the sand or clay fulgurites, the shape mimics the path of the lightning bolt as it enters the ground. All lightning strikes hitting the ground are capable of forming fulgurites, but not all lightning strikes will do so. A temperature of 1,800oC is required to melt sand and form a fulgurite, but this is not usually an impediment, since most lightning strikes have a temperature of 2,500oC.
In addition to the four main types mentioned above, there are the droplet fulgurites, which obviously resemble droplets, but, in composition, are similar to the clay and caliche fulgurites. Sand fulgurites tend to have rather fragile glass walls. Rock fulgurites are found not as discrete structures, but as veins or branching channels on a rock surface, or as a lining of fractures, which existed before the lightning strike.
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Deborah Painter (USA)