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Brazilian Agate

Most agate used today is from Brazil and Uruguay. Most is color treated right there, so getting untreated specimens is quite difficult. The Brazilian deposits go for about 400 miles along the border with Uruguay, with the bulk of the deposits being in Brazil. the source is a big basaltic lava "sandwich" of more or less porphyritic material. There is a quartzite layer sandwiched within the flows and this is a source for most of the agate. Nodules are of all sizes, with the largest measured at some 30 feet across, an amazing quantity of agate.

In natural form, the agates are mostly gray banded with some minor brown coloring as well. Bands of white material contrast nicely with the darker colors. On treating, the agate tends to take on the richer brown color of varying shades, and fine Brazilian agate is highly prized. The very finest agates, some 10% of the total take, are not treated and are used for the finest carving and gems. The Brazilian deposits have been known since 1576, but Idar Oberstein in Germany dominated the gem market at that time using local supplies till they were depleted.

Via The Tumbler 10.04, Rock Rollers 4/02, Rock Scoop 5/99 et al