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Describes common aspects of caves, their development, geology, animals that live in caves, and evidence of human culture left on the walls. (YDS)

Amber is the fossilized resin of now-extinct trees, primarily ancient conifers but also some flowering tropical trees. An aromatic, soft, sticky substance, resin in extinct trees probably served the same purposes as resin in modern trees: to protect the plant by sealing cuts and by excluding bacteria, fungi, and insects.

The article presents information on crystals. A crystal is the symmetrical, three-dimensional form assumed by a chemical element or compound during solidification of a gas or a liquid. "Crystal" comes from krystallos, Greek for "ice." Ancient scientists believed that quartz crystals were permanently frozen ice. Eventually, "crystal" came to apply to any flat-faced solid with a regular atomic structure. Crystallization can result from cooling of molten rock, precipitation from a solution, or even from freezing of water vapor. Crystal shapes can range from simple six-sided cubes to tongue-twisting hexoctahedrons, which boast 48 faces. Some substances crystallize into different shapes depending on growth conditions. Crystals expand by accumulating outside layers, not by growing from the inside as plants and animals do. Crystals can be undesirable such as when they grow inside human beings. Painful kidney stones are crystals of organic material such as uric acid, and gallstones are usually crystallized cholesterol. Liquefied quartz crystals, which retain many of the solid's properties, are used in appliances that require power-conserving, "liquid crystal" displays.