Trophy rocks

My first trophy rock is a 4 kg boulder of the Lavras conglomerate from the Tombador formation of the mid-Proterozoic Espinhaço Supergroup. This is a well-rounded, poorly sorted conglomerate with a clast size of 1-9 cm in this hand sample. The clasts are dominated by a pink arkose, with white and green arenites also present. The rock is clast- supported, and some clasts show evidence of minor pressure dissolution along their contacts. The matrix is slightly ferruginized. This rock is the purported source rock for the diamonds for the Lencois diamonds workings in Bahia state, Brazil.

My second trophy rock is a migmatite from the Wyoming craton. The strongly folded mesosome consists mostly of biotite and amphibole, with a minor opaque oxide phase. The leucosome follows the folds of the rock, but has equant, undeformed coarse-grained K-spar and quartz crystals, suggesting that crystallization was post-deformational.


The third trophy rock, part A, is a 45cm Cambrian stromatolite colony from an undisclosed location in Australia’s Georgoina basin. Part B is a smaller fragment of a contemporaneous stromatolite colony, with which has been progressively silicified, showing laminar color variations caused by trace elements in the chalcedony.

Unlike Brian’s rocks, these specimens are widely separated in space and time.