Ouro Preto
Ouro Preto
The Minas Gerais mining town and the principal source of imperial topaz
Ouro Preto is the colonial-era mining town in the south of Minas Gerais state, Brazil, and the principal source of imperial topaz — the reddish-orange to pink-orange variety that commands the highest prices in the topaz family. The town's name, meaning black gold in Portuguese, refers to the iron-stained alluvial gold that drove the eighteenth-century gold rush in the region; the topaz deposits emerged later, as the surface gold was worked out and miners turned to the weathered schists and stream gravels that hold the chromium-bearing topaz crystals.
Geological setting
The Ouro Preto topaz deposits lie in the Ouro Preto–Saramenha district of the Quadrilátero Ferrífero, a Proterozoic terrane of metasedimentary rocks intruded by granitic and pegmatitic bodies. Topaz crystallises in narrow quartz–topaz veins cutting altered phyllitic schists and is recovered both from the primary host rock and from the deeply weathered eluvial and alluvial deposits derived from it. The colour-causing chromophore is chromium, which substitutes for aluminium in the topaz lattice and shifts the spectrum from the colourless or sky-blue typical of pegmatitic topaz toward the orange-pink range characteristic of the Ouro Preto material.
Imperial topaz and related colours
The fine reddish-orange to pink-orange material from Ouro Preto is the type material for the trade name imperial topaz. The hue range runs from a saturated cherry-red through orange-red and orange-pink to a paler peach-pink, with the most valuable stones showing a clean orange-pink hue free of brown modifiers. A small fraction of the rough is naturally pink, and a small fraction is sherry-coloured. Yellow and golden-brown topaz also occurs in the district, but is typically priced below the orange-pink material.
Heat treatment can be used to convert sherry and certain orange material toward a more saturated pink — the so-called pinking heat treatment — and the pinking process is widely applied in the cutting centres. The trade convention is that fine, naturally orange-pink imperial topaz from Ouro Preto trades as untreated where laboratory documentation supports the determination, while pink material that began as sherry is disclosed as heat-treated. The disclosure regime is well established and is enforced by AGTA and CIBJO.
History and the colonial trade
Ouro Preto was the principal town of the Brazilian colonial province of Minas Gerais and the seat of the captaincy from 1721. Topaz from the surrounding workings entered the European market in the late eighteenth and nineteenth centuries, often by way of Lisbon, and the Portuguese royal house held a number of significant Ouro Preto topaz pieces. The provenance has been continuously worked since, with the most active modern operation being the Capão mine and the workings of Vermelhão.
In the trade
Ouro Preto is the global benchmark provenance for imperial topaz. Material from other localities — historic Ural Mountains stones, certain Pakistani topaz, and minor occurrences in Africa — competes only at the margins, and Ouro Preto remains the source of essentially all commercially significant fine imperial topaz currently entering the market. Sizes above ten carats are uncommon and command a strong premium; clean stones above twenty carats with fine orange-pink colour and laboratory confirmation of natural colour are rare and are priced accordingly, frequently above five hundred US dollars per carat for the finest material and considerably higher in exceptional sizes.
Treatment and identification
Pinking heat treatment is detectable in the laboratory by the absence of certain chromium-related features in the absorption spectrum and by colour-zoning patterns inconsistent with natural growth. AGL, GIA, Lotus Gemology, and the major European laboratories all offer treatment determination on imperial topaz. Origin determination at the Ouro Preto level is offered by some laboratories where supportable, but is less commercially significant than for ruby or sapphire because effectively all market-quality material originates from this district.
Care
Topaz cleaves perfectly on the basal pinacoid, and Ouro Preto material is no exception. The cutter and the bench jeweller must respect the cleavage; ultrasonic and steam cleaning are not recommended. Hardness at 8 makes the species durable in everyday wear, but a sharp blow at the wrong angle can split a stone along the cleavage. Bezel and protective settings are preferred for ring use.