Are there diamonds in meteorites?
Yes, but they are tiny. Meteorites contain nanodiamonds from ancient stars, diamonds in carbon-rich ureilites, and diamonds formed by the shock of impacts, none of them large enough to be gemstones.
Diamonds have turned up in meteorites in at least three different ways, and none of them would make a ring. The first kind are nanodiamonds, crystals only a few nanometres across, a few millionths of a millimetre. They were discovered in 1987 in primitive chondrites by dissolving away almost all of the rock with strong chemicals. Some carry unusual xenon isotopes that point to an origin in other stars, possibly in supernova explosions, though the exact source of most nanodiamonds is still debated. Tiny as they are, they are among the most abundant kinds of presolar material found in primitive meteorites.
The second kind occur in ureilites, an unusual group of carbon-rich achondrites. Their carbon appears as graphite with small diamonds, which most researchers attribute to shock from violent collisions on their parent body, though some argue that a portion grew under high pressure deep inside a large body. Diamonds were reported in the ureilite Novo-Urei, which fell in Russia in 1886, making it one of the first meteorites known to contain them. The Almahata Sitta stones, from the small asteroid 2008 TC3 that struck Sudan in 2008, include ureilites with diamonds that have been studied intensely.
The third kind formed when meteorites struck Earth. Fragments of the Canyon Diablo iron, from Arizona's Meteor Crater, contain diamond and lonsdaleite, a form of carbon with a hexagonal crystal structure, named in 1967 after the crystallographer Kathleen Lonsdale. Canyon Diablo was also where the French chemist Henri Moissan identified natural silicon carbide, later named moissanite in his honour.
So meteorite diamonds are real and scientifically fascinating, but they are microscopic messengers of stars and impacts rather than treasure.