I found this rock in a tropical climate river in Bolivia (specifically Ivirgarzama). It seems to have 5 or 6 layers.
- 1st layer is a slightly course matte rock layer
- 2nd layer is a more glossy rock layer
- 3rd layer is like the second layer but lighter
- 4th layer is this dark red coat rock layer
- 5th layer has a matte finish to it more sandy-grainy looking
- 6th layer definitely has a sandstone look to it
It's roughly 5 cm (2 in) high, 4 cm (1.5 in) across, and 15 cm (6 in) in circumference. I can scratch it with a knife blade so roughly a 5.5 in hardness. If possible could you also explain how this occurs in nature?