Uranus and Neptune are the two noted planets in our solar system where the atmospheric pressure is so intense that it theoretically rains diamonds according to professional astronomers.

Jupiter and Saturn also meet the criteria, but to a much lower level

According to Dr Kevin Baines, University of Wisconsin-Madison,

"It all begins in the upper atmosphere, in the thunderstorm alleys, where lightning turns methane into soot.

As the soot falls, the pressure on it increases. And after about 1,000 miles it turns to graphite - the sheet-like form of carbon you find in pencils.

By a depth of 6,000 km, these chunks of falling graphite toughen into diamonds - strong and nonreactive.

These continue to fall for another 30,000 km - "about two-and-a-half Earth-spans."

These are not big ones like the Hope Diamond, but about a centimeter in diameter.

http://www.bbc.com/news/science-environment-24477667

More Info: www.iflscience.com