Every gem fixed to every ring or necklace was forged deep inside our planet, according to its own recipe of elements, temperature and pressure.

Jade, for example, puzzled geologists for decades. If rocks contain jade, the scientists can be fairly sure those rocks are a vestige of an ocean buried underground. Rubies, on the other hand, appear in places where mountains formed from continental collisions, even if those mountains were eroded away millions of years ago.

So if jade came from ancient oceanic pressures, what about sapphires and rubies?

The Indian subcontinent is a good example of this. Originally a giant island, when plate tectonics crammed the subcontinent into Asia, the impact resulted in mountains hundreds of miles inland. Shale crushed within produced aluminum and oxygen crystals that eventually become sapphires.

But what happens where the crystals are pushed up towards the surface? If the rock such crystals are pushed through have chromium in them they pick up a red color, giving us rubies.

Via New York Times for the full article.