This is an odd one. The rounded crystal in the middle appears to be a chalcedony pseudomorph after calcite but it IS very fine-grained calcite. It is some sort of rounded crystal, complete with a trigonal pinacoid on the top. See photo. There are pure-white, elongated aragonaite crystals all around the calcite. There are also balls of spherical calcite on the specimen so, perhaps the original calcite crystal was overgrown with the finer-grained, almost botryoidal calcite? Whatever it is, it is cool!