There are two types of Jade; nephrite jade and jadelite. Both are formed by high temperatures and pressure within the earths surface. Usually this means raw jade is found in rough, mountainous regions.
Jade is found in many countries, including Siberia, China, Myanmar, United States, Guatamala, Indonesia, New Zealand, Australia and British Columbia.
Nephrite jade is primarily green in colour, however not always and will mostly appear darker in colour than jadeite. Even when highly polished nephrite jade will appear more opaque or matt and less lustrous than jadeite. There is white jade from China and Siberia, blue jade from the USA and visually black jade from Australia. Evidence has been found of past jade deposits, now depleted, in other countries.
Jadeite can be found in many colours, including green, lavender, orange and combinations of colour. Jadeite has a slightly different chemical composition to nephrite jade, is tougher and more durable.
The use of Jade and Jadeite for jewellery, ornaments and in some instances weapons extends back thousands of years to early civilisation. Jade is a fiborous stone, not a crystal gemstone, with its fibers laid in an interlocking fashion. Similar to that often seen in handmade paper. These characteristics provide jade with its strength and a fine grained surface which holds detail well when carved. This same fine grain ensures high quality jade will hold a high polish.
Connoisseurs of jade will seek examples free of fractures and inclusions, with almost pure colour and high translucency. Regrettably these are rare and beyond the budget of the average person.
Just because a stone is green does not mean it is Jade; though that doesn't prevent unscruplous traders from trying to tell you otherwise. Aside from conducting your own specific gravity test to determine if something is jade, your best protection is to purchase from a seller with demonstrable experience purchasing raw jade and jade products.
Read more on the how jade is classified.