Saniniu Laizer earned £2.4m ($3.4m) from the country’s mining ministry for the gemstones, which had a combined weight of 15kg (33 lb).
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.
“There will be a big party tomorrow,” Mr Laizer, a father of more than 30 children, told the BBC.
Its value is determined by rarity – the finer the colour or clarity, the higher the price. Mr Laizer mined the stones, weighing 9.2kg and 5.8kg, last week, but he sold them on Wednesday during a trading event in the northern region of Manyara.
Until now the largest Tanzanite rock to be mined weighed 3.3kg.
President John Magufuli phoned in to congratulate Mr Laizer on the find.
“This is the benefit of small-scale miners and this proves that Tanzania is rich,” the president said.
Mr Magufuli came to power in 2015 promising to safeguard the nation’s interest in the mining sector and increase the government’s revenue from it.
What did the new millionaire say?
Mr Laizer, 52, who has four wives, said he would slaughter one of his cows to celebrate.
He also plans to invest in his community in Simanjiro district in Manyara.
“I want to build a shopping mall and a school. I want to build this school near my home. There are many poor people around here who can’t afford to take their children to school.”
“I am not educated but I like things run in a professional way. So I would like my children to run the business professionally.”
Image copyrightTANZANIA MINISTRY OF MINERALS
He said the windfall would not change his lifestyle, and that he planned to continue looking after his 2,000 cows. He said he did not need to take any extra precautions despite his new-found riches.
“There is enough security [here]. There won’t be any problem. I can even walk around at night without any problem.”
Some small-scale miners like Mr Laizer acquire government licences to prospect for Tanzanite, but illegal mining is prevalent especially near mines owned by big companies.
In 2017, President Magufuli ordered the military to build a 24km (14-mile) perimeter wall around the Merelani mining site in Manyara, believed to be the world’s only source of Tanzanite.
A year later, the government reported an increase in revenue in the mining sector and attributed the rise to the construction of the wall, the BBC’s Sammy Awami in Dar es Salaam reports. (H/T BBC)