Grass Block

Grass is a common block that is on the ground on newly generated maps. It looks like a dirt tile with grass on, and is pretty much that. It requires sunlight to continue to exist and will revert to dirt slowly if sunlight is removed. Dirt will also change to grass over time if there is another grass block next to it and it's receiving sunlight.

Grass and dirt changing between each other is a common cause of chunk updates.