Geraisites are the first Tektites ever recognised in Brazil. Residents of northern Minas Gerais know this black glass well, but its significance only emerged when people from two villages independently sent specimens to geologists for analysis. In November 2023, Professor Álvaro Penteado Crósta of the University of Campinas (UNICAMP), Gabriel Silva of the University of São Paulo and Ludovic Ferrière of the Natural History Museums in Abu Dhabi and Vienna searched the region themselves and recovered ten specimens in the field across a distance of about 90 km, mainly around Taiobeiras, Curral de Dentro and São João do Paraíso. The team named the glass after the state of Minas Gerais, first presented it to the Meteoritical Society in 2024, and published the full description in the journal Geology in December 2025.

Tektites form when a large meteorite impact melts rock at or near the Earth’s surface and throws the molten material hundreds or even thousands of kilometres from the crater. The glass cools as it flies, which is why Geraisites take the classic splash-form shapes: spheres, ellipsoids, teardrops, discs, dumbbells and twisted forms. Most are less than 5 cm long, and the largest recorded weighs 85.4g. Almost all are black with a heavily pitted skin, and thin edges glow olive green to brown when held to a strong light; only a single olive green specimen has been reported. Analyses give a silica content of 70.3 to 73.7 wt% and a water content of just 70 to 110 ppm, far lower than in volcanic obsidian, along with rare inclusions of lechatelierite, a silica glass that forms only at extreme temperatures. Together these features confirm an impact origin.

The best current estimate of their age, from argon-argon dating, is about 6.3 million years, in the late Miocene, although the authors note that inherited argon may be present and further analyses are needed to fix the age precisely. Strontium, neodymium and hafnium isotopes suggest the melt came from very ancient continental crust, most likely Mesoarchean felsic rocks. No source crater has yet been found. Geraisites now join the Australasian, Central European, Ivory Coast, North American and Belize Tektites as one of only a handful of Tektite strewn fields known on Earth, and the field is still growing. According to Agência FAPESP, more than 600 specimens had been collected by early 2026, and new finds in Bahia and Piauí have extended the known area to more than 900 km.

For collectors, Geraisites offer the chance to own material from a Tektite strewn field first described in the peer-reviewed literature in December 2025, with a clear discovery story, a published age and an active research programme behind it. This specimen is a complete individual, preserving its natural pitted surface.

Further reading

• Crósta et al. (2026), Geraisite: The first tektite occurrence in Brazil, Geology 54(2): 163–167
• Supplemental material for the Geology paper: methods, specimen list and data tables
• Crósta et al. (2024), A new tektite strewn field discovered in Brazil: Geraisites, 86th Annual Meeting of the Meteoritical Society (PDF)
• Crósta et al. (2024), Geraisitos: a primeira ocorrência de tectitos no Brasil, 51º Congresso Brasileiro de Geologia, Belo Horizonte (in Portuguese, PDF)
• Agência FAPESP (2026), Impact-formed glass evidence of cosmic collision in Brazil about 6 million years ago