The following is by Tualatin Valley Rock and Gem Club member Taylor Hunt. if you would like to contribute to our newsletter and website with articles on various rocks, minerals, and gems, please contact us.
The gemstone jadeite generally forms as a result of the plate tectonic process of subduction. Jade made from jadeite forms when supercritical fluids from subducting oceanic crust condense in the overlying man-tel wedge (the “wedge” is all the sediments swept up, piled and squeezed between the subducting oceanic plate and a continental mass), between 20 & 60 km deep in the Earth. Jadeite deposits thus mark the loca-tion of exhumed fossil subduction zones.
A new term PTG’s, plate tectonic gemstone, and jadeite is one PTG currently recognized. For various reasons most PTG’s are found in rocks or continental plates considered young to planet Earth. Most are no older than the formation of the supercontinent of Rodinia or 1,000 ma yr. Petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion. Recognition of the PTG’s links modern concepts of plate tectonics to economic gemstones deposits and the ancient concepts of beauty, and may aid in exploration for new deposits.
Any mineral or stone beautiful enough to be sought, mined and sold for its beauty alone is a gemstone. The subclass of rocks and minerals that comprise gemstones—whether precious or semi-precious—has mostly been established since antiquity. Humans have sought and prized gemstones since thousands of years be-fore the science of geology was established. Because gemstones are rare by definition, the geological conditions that produce them must have been exceptional. Thus, there is a confluence of economic, aesthetic and academic interest in gemstones. Jade — specifically the variety jadeite — is the characteristic beautiful product of normal oceanic lithosphere subduction.
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Jade is a term ascribed to two different materials with similar properties, toughness, and beauty that evolved in usage and significance from toolstones for axes, choppers and hammers to one of the most highly revered gemstones in the world. As a tool, jade was employed during the Paleolithic (stone age, before 3500 BCE) but was raised to high symbolic stature as a gemstone in proto—Chinese Hongshan and Liangzhu cultures by 3500 BCE, and in the Jom’on culture of Japan by 3000 BCE, and in Central America by the Olmec of the Early Formative period by at least 1500 BCE and later in the Mayan civilization. Hard jade (jadeitite) or “ying yu” in Chinese consists predominately of pyroxene minerals, jadeite (Na,Al,Si2,O8), while soft jade (nephrite jade) “ruan yu” come from amphibole minerals of tremolite-actinolite [Ca2(Mg,Fe)5,Si8O22(OH)2]. The term jade was derived from the Spanish “piedra de yjada” (loin stone) for talismans worn by the Aztec to ease abdominal pain, but was mistranslated to the word jade. In New Zealand, nephrite jade is sometimes called greenstone and was a favorite of the Maoris. Continue reading “Jade: Did You Know There are More Than Two Types?”