The Nature Core is a block added by Random Things. It naturally generates inside a structure consist of Jungle Leaves and Jungle Log in a checkerboard pattern in a 3x3x3 area centered around it.
Every tick, there is a chance for one of the following independent (multiple different ones could happen in the same tick) effects to occur:
1⁄40 chance to replace a random sand block within a 5x3x5 area with grass if there is an air block above or dirt if there isn't an air block above
1⁄400 chance to spawn an animal (dependent on the biome type of the selected block) facing a random direction within a 5x3x5 area if there are isn't more than one animal within a 5x5x5 area
1⁄100 chance to force a growth state within a 5x3x5 area
1⁄600 chance to plant an oak sapling within a 10-20 block radius in an empty or replaceable spot on valid soil
1⁄600 chance to grow jungle leaves and wood to rebuild it's original generated structure
It can only be harvested with a Diamond Axe or better. (Mining Level Obsidian)