-There are certain non-renewable "root" resources, these are only obtained by player finding them from the world.
(examples: Gravel, Nether Quartz Ore, Diamond Horse Armor.), these resources can't be made from other resources. (clay is weird, because it is an root resource but has 2 forms that can be converted into each other.)
For example: Dirt isn't an "root" resource, because it can be made from coarse dirt that is made from gravel.
No non-renewable item needs more than 1 kind of "root" item to make, some things are made from 2 different non-renewable resources
(Nether Quartz + Andesite = Granite), but all the items used to make the product have the same "root" item. (Nether Quartz Ore for Andesite and Nether Quartz.)
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No, but you don't need dirt for every new generated grass block. If you have for example a 3 by 3 field with grass and you turn some of those blocks into to dirt, the grass will again spread on them.
So the dirt is not renewable, but the grass is.
But grass blocks as well as podzol, mycelium and farmland are variants of dirt blocks, so really you are just switching a non-renewable resorce, dirt, from one form to another.
Coarse dirt is a special case since it requires an additional resorce, gravel.
It sounds reasonable at first (and even second) glance, but when you look at it more closely it's no different than saying diamonds are renewable because you can get them by splitting diamond blocks (which you get from combining diamonds).