nice project.
I actually like the generall atmosphere of the textures, although it is somewhat dreary or at least the way I use stone blocks for my buildings appears such.
I would suggest to have all the licence and info files within the texture folder as it gets installed in minetest so that it doesn't get lost and can easily be refered from within an at some point in the future then old minetest installation.
(I do have some old texture packs installed of which I have no further information except for the folder name.)
Below is a picture of the different stone types within Minetest Game.

from left to right:
desert stone
sand stone
stone
desert sand stone
silver sand stone
and from top to bottom:
stone brick
stone block
stone
I would suggest to swap default_desert_stone_brick.png with default_desert_stone_block.png and likewise default_sandstone_brick.png with default_sandstone_block.png
The default_sandstone texture itself might be improved by a little more contrast as the texture merges to a very uniform grey from allready a short distance away.