I keep getting asked this question, how does it work? Let me tell you something out of the box.
The main center for gem and mineral trading is Arusha at the foot of the mighty volcano Mt. Meru. Minerals are also traded in Dar es Salaam, the most important city in Tanzania, and to a lesser extent in Moshi at the foot of Mount Kilimanjaro.
The following is important to know: No stone may leave the country without valid export documents. These documents can only be obtained from a so-called "master dealer", only these dealers have an export license. It is now also forbidden for foreigners to purchase minerals directly in the discovery areas, so the only way is through the master dealer. They sell minerals (or polished stones) from their property or brokers come to their office and bring the minerals from the discovery areas. The brokers also need a state license, they are the mediators between the miners or the fund areas and the master dealers. This is one of the main reasons why it is almost impossible for foreigners to acquire good minerals in the discovery areas; these are immediately brought to the trading centers by the brokers.
What are we going to do? We also visit the find areas, but more out of scientific interest. In Arusha we rent the office of a master dealer. There we are then offered the minerals, mostly by the brokers. Since we have now been working there for more than 20 years, we have been able to build up a good network and have become very well known over the years. Before we leave for Tanzania, we announce our arrival well in advance so that the brokers can pick up minerals for us.
The brokers are let into the office individually, they unpack their stones and we select the minerals that we like. Mostly Swahili is spoken, as only very few of the brokers speak English. The price negotiations are often very amusing. A mostly extremely high price is demanded and only after long, tough negotiations do you come closer to the common ideas, but this often does not succeed. The brokers often leave the room briefly to telephone the owners of the stones. Rarely does the actual owner come into the office, but mostly they wish to remain anonymous.
We are mainly offered minerals from Tanzania, but also minerals from DR Congo, Malawi, Mozambique Zambia and even Namibia.
In terms of time, we usually stay in Arusha for one to two weeks, then the interesting replenishment stops and we often see plays for the 5th time that we have already rejected 4 times. A list of the acquired minerals is now drawn up, with their weight and value. On this basis, the master dealer creates an invoice that is generally kept relatively low, since the government raises "royality" on the minerals to be exported. This amounted to 5% of the invoice value until mid-2017. The minerals are packed in a tin box and with this and the bill it goes to the mining ministry ("madini"). 
The employees check the bill and up to the middle of last year it was mostly accepted. You pay the 5% to the government treasury and a small donation to the staff of the madini and then the tin box is sealed, like in the old days with a wax seal. Once this has happened, the box may no longer be opened within Tanzania (not even by the security guards at the airport). 
Documents from the tax authorities and customs are then required, these are usually issued without any problems. Now nothing stands in the way of leaving the country.
It was like that until mid-2017. Then it was drastically tightened. During my last visit in November 2017, every stone was unpacked and weighed. The prices shown on the invoice have been drastically increased, zTabstrus increased. For example, 200 grams of small, insignificant tanzanite crystals were upgraded to $6000 instead of $200, and not only 6% royality had to be paid on these sums, but also 1% inspection fee and 6% handling tax. 
In retrospect, one can say at the time that none of this was so bad. In the summer of 2017, a law was passed that prohibits the export of rough stones (unfortunately, this also includes collector minerals). This law came into force at the beginning of December 2017. Since then, the madini no longer seals, which means that no rough stone can legally leave Tanzania. Nobody can explain this. All brokers and small traders are cut off from any earnings (some we support from here), mining has stopped. We will be back in Arusha in July 2018, if the restrictions are not lifted things will look bad for new interesting minerals from beautiful Tanzania.
In the meantime "normal" conditions have returned to Tanzania. After a phase in spring 2019, when the sealing in Dodoma (about 450 km south of Arusha) was carried out and lasted about 1 week, the mining authority in Arusha is now working again and sealing the minerals as before.
The situation in Mererani, the tanzanite mines, has changed drastically. The mining area is now surrounded by a huge metal fence, guarded by the military and numerous cameras. A gate is the only entrance and exit and everyone who wants to leave the area must show their stones there. The mining authority and tax authorities are present at this gate, all stones are valued and the royality of currently 7% must be paid immediately . Physical examinations are carried out, it is also reported that women have to undress completely (although men do too). This greatly complicates the work of brokers. they now have to pay in advance without knowing whether they can also sell the minerals.
Since October 2019, the Tanzanian government has taken rigorous measures to reduce smuggling and thus tax evasion on minerals. In the center of Arusha, an area of approx. 10,000 square meters was fenced in and minerals may only be traded on this site. This applies to both foreign and domestic persons. All master dealers are required to have an office there, and only in a master dealer's office can foreigners purchase minerals. As a foreigner, you must find a master dealer who will apply to Madini for permission to purchase stones. Once this has taken place, you may only buy minerals in his office and only from him, ie contact with the brokers, which was previously unproblematic, is now prohibited
Since February 1st, 2020, brokers have only been allowed to enter the said area with a valid broker license and most of them do not have this because they do not have the money to pay the relatively high costs. The sales area is more or less empty and thus the mineral trade has come to a standstill.