well, anon, it's because jade is very awesome.
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Jade is a stone valued for its beauty and utility. The term is actually applied to two different stones: jadeitite and nephrite. Both of these stones exhibit very similar characteristics, having roughly the same hardness, color range, and ability to be carved. Jadeitite is the rarer stone, and as such tends to be considerably more expensive.
This stone appears historically throughout the world in various cultures, but is perhaps most well-known as the stone of China. Jade has been used in China dating back nearly seven-thousand years, used in both decorative carvings and more practical items. Throughout China’s history, it has been important, both to common people and to the various rulers.
Mythically, the highly-valued emerald-green jade was said to literally be moonlight in crystal form. With the establishment of the Chinese dynastic system, jade became known as the Imperial stone, being used as the material for amazingly ornate objects, including the iconic armor some rulers were buried in. Only nephrite is commonly found in China, but beginning in the 19th century jadeitite was imported from nearby Burma, and became the variety of choice, known as Kingfisher jade.
Jade was also important in the historical Americas, where it was quite rare, and therefore quite valuable. Many ceremonial objects were crafted from stone quarried from modern-day Guatemala, and it was also used as a medium upon which to inscribe hieroglyphs. Although no culture will ever truly compete with the Chinese for the intricacies and beauty of their carvings, many of these Mesoamerican artifacts are truly stunning.
In many parts of the world, jade is viewed as a stone symbolizing luck, and thought to offer protection. Amulets made from it can be found in more than one culture, as can protector figurines. The Egyptians believed it embodied personal harmony, and used it in their art accordingly.
Chemically speaking, nephrite jade is made up of a mineral called actinolite, a mineral high in both magnesium and calcium. It is quite hard, and it is likely that many early cultures began valuing it so highly because it proved an ideal material from which to craft weapons and tools.
The value of jade varies widely, depending on the tastes in a region, and the history of the stone itself. Tastes for jade are different in different cultures, and the costs will reflect this. In many Western countries, for example, the most highly-prized varieties are those with the most vivid greens — especially those which seem almost emerald in hue. In China, on the other hand, jade that is virtually white, or with tinges of yellow, is very valuable.
When looking to buy jade, it is important to make sure you are actually getting what you set out to buy. While the term may reasonably be used to describe either nephrite or jadeitite, it is often also used to refer to serpentine, Vesuvianite, green quartz, carnelian, and other stones. Because of the range of issues that need to be taken into consideration, and because of the many different, lesser stones sold as jade, it is highly recommended that it be purchased through a reputable dealer, unless you are confident in your own abilities to appraise the stones.