Recent research shows that there may be diamonds on Jupiter and Saturn.
On Jupiter and Saturn, scientists have recently discovered that there could be a lot of carbon on the planet. This carbon could be present in the form of diamonds that rain down from the sky. While the diamonds eventually melt and don’t stay around for long, they can be as large as a centimeter across.
How do these diamonds form to begin with?
In the upper atmosphere of Saturn and Jupiter, methane is struck by lightning in storms and is turned into soot. This soot then falls down, transforming into graphite and finally diamond! However, because there is such high pressures and temperatures closer to the cores of these gas giants, the diamonds melt. On Neptune and Uranus though, the colder temperatures allow diamonds to stay solid. This research is very new so it is not peer reviewed yet, but it is likely that there are diamonds on Jupiter and Saturn.
Forty light years outside of our Solar System, there is a planet called 55 Cancri e that is more carbon rich than Earth so it may be made of mostly diamond! It takes quite a bit of heat and pressure on Earth to form diamonds, but on 55 Cancri e, the carbon richness makes it an ideal place for diamonds to form.
Holly's Astronomy Blog 
The gas giants arnt the only places in the solar system with what we consider to be precious materials! Asteroids also contain many heavy metals which are rare on Earth. In fact, there is so much raw material in asteroids that mining them would produce an over-supply of these metals on Earth, causing their prices to plummet and then they would no longer be precious!
The diamonds melt? That’s the saddest thing I’ve ever heard, though I don’t suppose we could get to them anyway, and it’s not as if they’d emerge cut and polished like the kind we see in a Cartier or De Beers advertisement. All the same, it seems a shameful waste :[ But I think it’s awesome that lightning is involved in their production. Lightning makes a beautiful, shiny creation here on earth as well–when it strikes the sand on a beach, beneath the surface of the sand it creates a gorgeous, organic glass statue. It melts the sand (which is ground up glass anyway, among lots of other things) and the form of the branches of the lightning are captured in the melted glass, which then cools, encased in sand.