When lava pools generate in a forest or taiga biome, a nearby tree's leaves can become stone.

How to recreate: Find a taiga or forest biome, and a lava pool. if there is a close enough tree, some of the leaves become stone, granite, andesite or diorite.

this could be fixed if the lava pools were generated after trees, or if leaves were excluded from being replaceable blocks for the pools of lava.

Edit: provided seed, world specs and coords.

superflat specs: 2;7,59x1,3x3,2;5;decoration,lava_lake

coords: 337x,66y,442z

seed: -3735310115214278708

World is superflat, its hard to find a default world example. they do exist though.