The area near to the river Rio Negro became famous by its finds of fossils of gigantic Sauropodes. Some of the fossil dino-bones from the cretaceous time are beautiful agatized and are presenting (similarly as the Dino-bones from Utah) a tangle of agatized cells of former tube bones branched out finely. Fine formated fortification agates within the range of the previous caves are sometimes situated in the center of the bones.
The “Lapis-Magazine” did report about findings of complete agatized dinosaur eggs from the Rio Negro area (extra Lapis # 19 "agate", page 78). Distinctive for these football-shaped large eggs of the biggest Sauropodes is their peculiar formation of the shale (small elongated columns).
Interesting is also the occurrence from well formed Pseudomorphs after the mineral aragonite. The pseudomorphs perfectly replace the aggregates by chalcedony, on the inside unfortunately they are showing no fortification agate, only confused chalcedony.
Beautiful jaspers with strong contrasts and intensive colors originates likewise from the banks of the Rio Negro.