Agate slice 168mm showing two quartz cavities with crystals from Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, South region, Brazil. A great insight with the contrasting bands representing the growth that has taken place over millions of years. Agate is banded chalcedony, a form of microcrystalline quartz made up of twisted crystal fibres. Forming within the cavities and veins of lava, Agate replaces other mineral deposits and limestones and mostly found within volcanic lavas.
Rio Grande do Sul, north of Uruguay is considered to be one of the best locations for finding an agate slice in Brazil and the rest that follow are the States of Santa Catharina, Parana, Sao Paolo and Minas Gerais. Two Agate slice producing areas are in the States of Bahia in East Central Brazil, and one other in state is Roaraima in Brazils extreme Northwest.
The most explosive volcanoes produce ashes or tuffs, creating a mixture of ash, lava and sediments which are torn from the surrounding rocks. It's thought that micro-shards of silica in glass form produced by erupting volcanos which produce ash and tuffs, eventually release silica that permeates through old underlying lava cavities through atmospheric and rain water and can also include decomposing of vegetation and animal matter within the sediments deposited between eruptions.