A Tanzanian miner who made headlines in June for discovering two rough Tanzanite stones valued at $3.4m sold another one for $2m on Monday morning.
The third discovery by Saniniu Laizer, who is a small-scale miner, weighed 6.3kg.
There was a lot of excitement from members of his community in the build up to the sale and hundreds were waiting from early morning to witness it, the BBC's Aboubakar Famau reports from the capital, Dodoma.
Many other small-scale miners were shocked by Mr Laizer's luck but also acknowledged that perseverance may have something to do with it, our correspondent adds.
Mr Laizer has promised to use the money from the finds to build a school and a health centre.
Tanzanite is only found in northern Tanzania and is used to make ornaments.
It is one of the rarest gemstones on Earth, and one local geologist estimates its supply may be entirely depleted within the next 20 years.
The precious stone's appeal lies in its variety of hues, including green, red, purple and blue.