Chalcedony is a variety of the silica mineral quartz and is usually found mixed with other minerals in rocks like agates, onyx, carnelian, tiger eye. etc. It was prized by the Romans who used it for cameo and intaglio carvings.
Chalcedony Rosettes are not quite stones and not quite crystals - more like small, interesting globs of mineral matter. Each is about the size of a half dollar and 1/4 inch thick.
This rosette is from Rio Grande do Sul, Brazil .
1 Chalcedony Rosette