But hundreds of small-scale, unlicensed pit mines have sprung up all over the area—bringing with them many of the same problems that have plagued artisanal mining of other minerals elsewhere: unsafe working conditions and exploitative labor practices.

Because none of the miners were licensed, miners had to sell the uncut gemstones they excavated illicitly, giving rise to an underground economy of smuggling networks, loan sharks, and organized crime. It also spurred government corruption as officials were paid to look the other way. Meanwhile, Tanzania itself lost out on revenues it could have derived from royalty fees and mineral rights. The government estimated in 2017 that 40% of tanzanite was being smuggled out of the country.

These sorts of problems are common to artisanal mining, according to a 2017 report of the Intergovernmental Forum on Mining, Minerals, Metals and Sustainable Development (IGF). “Environmental and health and safety practices tend to be very poor. For example, dust and fine particles resulting from blasting and drilling cause respiratory illnesses,” the IGF wrote.

Small-scale gold mining, which uses mercury to separate gold from other metals, is especially problematic. In fact, artisanal mining is the largest global source of mercury pollution—estimated at 1,400 tonnes of mercury released into the environment each year, the IGF says. “Exposure to mercury can have serious health impacts, including irreversible brain damage,” the report continues. “Mercury is also difficult to contain and can be toxic at even very small doses. It can be transported long distances by air or water, poisoning the soil and waterways, and eventually making its way into the food chain. In sub-Saharan Africa, most of these risks are borne by women.”

And yet small-scale mining is key to the production of many valuable minerals: 20% of the world’s gold supply, 80% of sapphires, and 20% of diamonds are mined this way, according to the IGF. Key metals for electronic components are also extracted by small-scale mines: 25% of the world’s tin and 26% of the supply of tantalum, which is used in computer chips and smartphones.

In the case of tanzanite, the Tanzanian government first tried to wall off the problem—literally. Because the area where tanzanite is found is concentrated so densely, in 2017, Tanzania’s President John Magufuli decided to build a fence around the entire mining region. But fences are never very good at separating supply from demand. The fence did reduce the amount of tanzanite being mined illicitly, but it also drove prices higher and made smuggling more lucrative.