Rock with number 23

This collectible works the same way as dogtags.

Location
The rock is very obvious and cannot be missed - it is at the beginning of chapter 6 (Beyond the void). The collecting part is very easy as well. At the repulsor that is blocking the way to reach the last dimensional portal, there is a gravitation platform. Then you jump to the left platform, then to the right one with the moving block elevator. If you fall to the lower platform, start again. Then after you reach the moving block elevator, jump in the gap on the very left, but when jumping in the gap make sure you are more to the right side, otherwise it´s your death.

If you did everything correctly, you should land on the small platform with the rock.

Usage
Unlocks a Steam achievement. Other than that it is useless.