From estate sale of rock collector, this specimen from Crosby Mines in 1956 Minnesota measures approximately 3" tall, 3" wide, and thickness ranges from 2 to 1 1/4".  I am not sure what the composition is - a mix with some jasper?  Mounted on 5" square, 1/2" thick slab of onyx (?).  There is a build-up material underneath rock (whitish substance painted dark).