Jade is really two kinds of metamorphic rock, jadeite and nephrite, that look pretty much exactly the same. Scientists can tell them apart using chemical tests and x-rays. Like quartz, both jadeite and nephrite are chains of silicon and oxygen atoms.
Jadeite is a simpler chain and nephrite is a double chain. A trace of iron in the jade makes it look green (the same way iron makes the blood in your veins look bluish). Sometimes chromium makes the green color instead.
Jade forms where two tectonic plates meet and there’s a lot of pressure under the earth. The pressure compresses a rock called pyroxene and transforms it into crystals of jade. There’s a lot of jade in China, and artists in China used jade to make sculptures and jewelry from the Stone Age right up to modern times.
In Europe, jade was very rare and expensive. Doctors in Spain in the 1500s AD thought putting jade stones on your kidneys would help cure kidney disease. Our word “jade” comes from the Spanish word for kidneys, and “nephrite”, another word for jade, comes from the Latin word for kidneys (because some fancier doctors used the Latin word). In New Zealand, where jade was also rare, Maori people used jade as a kind of sacred money, so that if you cheated someone on a deal using jade, you’d be cursed.