Minas Gerais, a large inland state in southeastern Brazil, is one of the richest gem regions in the world. Its name means "general mines," a reminder of the gold rush that opened it up around 1700. The state is the classic source of imperial topaz from Ouro Preto, produces much of Brazil's aquamarine and tourmaline, gave the world a major new diamond supply from Diamantina in the 18th century, and yields emerald near Nova Era. The city of Teofilo Otoni is one of the busiest colored stone markets anywhere.
Gold, diamonds and the colonial era
Portuguese settlers and prospectors found gold in the hills of Minas Gerais in the 1690s. The rush that followed drew huge numbers of people, including enslaved Africans forced to work the mines, and built towns such as Ouro Preto (then called Vila Rica). Gold from Minas paid for churches, palaces and much of Portugal's spending in the 18th century.
Diamonds were found in the 1720s in the streams around Arraial do Tijuco, the town now called Diamantina. Brazilian diamonds soon replaced India as Europe's main supply. The Portuguese crown declared a restricted diamond district, controlled mining tightly and later ran the trade as a royal monopoly to keep prices up and stop smuggling. Many of the stones were cut in Amsterdam and other European centers. Brazil remained the leading diamond source until the South African finds of the 1870s. See diamond mines of Brazil for more on the deposits.
Geology of a gem state
Minas Gerais lies on some of the oldest rock in South America. Over long periods, mountain building, metamorphism and granite intrusions created a wide range of gem-forming settings:
- Granitic pegmatites in the east and northeast, the source of aquamarine, tourmaline, topaz, morganite and kunzite.
- Hydrothermal veins in metamorphic rocks near Ouro Preto, where imperial topaz forms.
- Schists in contact with granite and pegmatite, which host emerald.
- Old conglomerates and river gravels that carry diamonds.
The igneous rock and metamorphic rock pages explain how pegmatites and metamorphic gem deposits form.
Ouro Preto imperial topaz
The hills around Ouro Preto are the classic source of imperial topaz, a golden orange to pinkish orange and sometimes red to violet variety. The name is often linked to the Brazilian imperial family of the 19th century, though the exact origin of the term is uncertain.
Ouro Preto topaz occurs in weathered veins in the local rocks, unlike the pegmatite topaz found elsewhere. Crystals are usually prismatic and often not very large. Some pale orange material is heated to turn it pink, a long-standing practice in the trade. Fine natural imperial topaz in strong colors is uncommon, and Ouro Preto has been the main commercial source for centuries.
Aquamarine
Minas Gerais has produced much of the world's fine aquamarine. The pegmatites of the northeast, in valleys around Teofilo Otoni, Marambaia and Padre Paraiso, have yielded large crystals, some weighing many kilograms.
The trade term "Santa Maria" refers to an intense, saturated blue first associated with aquamarine from the Santa Maria de Itabira area. The name is now used loosely for aquamarine of deep color from any source. Much aquamarine is heated to remove greenish or yellowish tints and leave a purer blue.
Tourmaline
Minas Gerais is one of the most important sources of tourmaline. The pegmatites of the northeast, around Aracuai, Governador Valadares, Conselheiro Pena and other towns, produce green, blue, pink, red and multicolor crystals, including watermelon tourmaline with a pink center and green rind.
The Cruzeiro mine near Sao Jose da Safira, worked through the 20th century, has been a famous source of fine pink and green tourmaline. Some of the rubellite and green tourmaline in the market comes from Minas pegmatites. The neon blue copper-bearing tourmaline known as Paraiba tourmaline was found in the neighboring state of Paraiba, not in Minas, though related deposits were later found elsewhere in northeastern Brazil.
Emerald at Nova Era
Emerald deposits in Minas Gerais lie mainly near the towns of Nova Era and Itabira. The Belmont mine, opened after emeralds were found in the late 1970s, became one of Brazil's best-known emerald operations. The emeralds form in schist where the rock came into contact with beryllium-rich granite and pegmatite fluids.
Brazilian emerald from this area tends to be a lighter, slightly yellowish to bluish green, and it often has fewer inclusions than some other sources. Most of the material is commercial grade, and fine stones are uncommon.
Teofilo Otoni gem market
Teofilo Otoni, in the northeast of the state, is a major trading town for Brazilian colored stones. It sits close to many pegmatite mining areas, and rough from the surrounding region flows through its dealers' offices. Buyers come from around the world, and the town has hosted an international gem fair for many years.
Many dealers, cutters and exporters in Teofilo Otoni have family roots in the trade going back generations. Some have ties to the German gem town of Idar-Oberstein, whose emigrants helped build the Brazilian gem trade from the 19th century.
Other gems of Minas Gerais
Minas pegmatites and veins also produce morganite, kunzite, chrysoberyl and alexandrite, garnet, and many varieties of quartz. Amethyst and agate also come from Brazil in large quantities, mostly from the southern state of Rio Grande do Sul.
Frequently asked questions
What gems are found in Minas Gerais?
Minas Gerais produces imperial topaz, aquamarine, tourmaline, emerald, morganite, kunzite, chrysoberyl, alexandrite, garnet and quartz, and historically produced large amounts of diamond.
Where does imperial topaz come from?
The classic source is the area around Ouro Preto in Minas Gerais, Brazil. The topaz there forms in weathered veins and is known for golden orange to pinkish orange colors.
When were diamonds found in Brazil?
Diamonds were found in the 1720s near the town now called Diamantina in Minas Gerais. Brazil then became Europe's main diamond source until the South African discoveries of the 1870s.
Is Paraiba tourmaline from Minas Gerais?
No. Paraiba tourmaline was first found in the state of Paraiba in northeastern Brazil. Minas Gerais is known for other tourmaline colors, including pink, green and multicolor.