Years ago in Brazil, an unfaceted Emerald weighing 1,900,00 carats was mined and found to have an estimated value of $400 million. Of course, the owner decided to sell it on eBay instead for less than 1/4th of that, but the point is that gemstones can get very large. Of course, faceting a gemstone that size would be pointless.
However, probably for the sake of setting a record, some of the larger gemstones have been faceted and cut. A large slab of Yellow Topaz sits at nearly 23,000 carats, weighs 26lb and is on display at the Smithsonian. There are enormous blocks of Emerald, a different Topaz that was shaped into a human-sized marble and strange, statue-sized blocks of pearl pulled out of the ocean. But the winner goes to a particular diamond.
The Golden Jubilee Diamond
In 1985, miners in South Africa pulled a 755 carat brown diamond out of the top mine in the country. An underground lab was constructed specifically to work on this diamond and it took two years to carve it into the gemstone it currently sits as.
By the end of their work, it had gone from 755 carats down to 545. After that, it would change hands a few times in various acquisitions. Today, the crown jewel sits in the Royal Museum at the Pimammek Golden Temple Throne Hall in the country of Bangkok.
You can see photos of the gemstone here.