Tanzanian President John Magufuli on Friday inaugurated a 24km wall around the country's tanzanite mines to prevent smuggling of the precious violet-blue stones, which are unique to the East African country.Magufuli in September ordered the army to build the wall around the mines, located in the Mererani hills near Mount Kilimanjaro.Several metres high, the mustard-yellow wall has only one entrance, which is secured by the army. It cost $2.2m, according to official figures."Before the construction of this wall, there was a lot of tanzanite being lost, about 40% of all production," Magufuli said during a ceremony transmitted on national television, referring to miners smuggling out the gems.He highlighted a 2017 decision that from now on the wholesale of tanzanite must be carried out under the control of the Tanzanian Central Bank inside the new wall.Prior to this decision, tanzanite was sold in Arusha and Nairobi.The wall is the latest move by Magufuli - who swept to power in 2015 on an anti-corruption platform - to regulate the mining sector, which has faced allegations of fraud and underreporting of production and profits.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. A commission of enquiry set up by Magufuli estimated in June that $90bn had been lost in tax evasion arising from mining operations since 1998.A parliamentary report published in September found that corruption and unfavourable contracts meant that the craze for tanzanite was mainly benefitting smugglers and mining operators.Magufuli has locked horns with foreign mining companies, accusing them of under-valuing their production of gold, diamonds and tanzanite, resulting in a loss of billions of dollars in taxes and royalties since 1998.Tanzanite was first found in the foothills of Kilimanjaro in 1967, although the discoverer is a matter of some dispute. The Tanzanian government recognises a Maasai herdsman named Jumanne Ngoma who stumbled upon the distinctively-coloured gem; there are rival claims for another Maasai tribesman, Ali Juu Ya Watu, and a local prospector named Manuel de Souza.