I fully agree, having only those “default” stones is very boring.
This is something I have been waiting for a very long time.
Adding more stone types would enhance the gameplay, especially if they come in different hardnesses. For example, one gameplay effect might be that your tools might be to weak to go through an entire large area of stone, so you have either get better tools or find your way around this. The exploration of the underground would become much more interesting. In Minetest Game, you basically only need to dig down. With only default stones, you basically just need one tool to go down to the lowest depth. In Minetest Game, even the wooden pickaxe is enough to reach the lowest depth, it just takes a lot of time (and wood :D).
IMO this would change gameplay significantly and it can't be done by simply changing Minetest Game. The boring “default” stones are way too ingrained in Minetest Game that they could be ripped out easily. Therefore, it would make sense to base an entire new subgame around this concept.
All mods which have been shown so far do not really provide what has been requested.
There are some mods which add 1 or 2 new stone types but that's usually it. As has been said, darkage only spawns stones like ores. This is clearly not what I want. Stones should not be just like ores with a different texture. There should be large areas or even complex layers of stones. Think of “3D biomes” for the underground.
So basically what we need are 3 things:
1) Defining a large set of new stones with their own unique properties, especially in hardness
2) Providing a set of tools to dig through and to give a sense of progression
3) Generating the stones to create a complex underground of various stones
To my knowledge, we currently do not have enough mods to complete this task. The mods for this have yet to be written or are very, very imcomplete.
There is an interesing very experimental mod for adding new rocks and making a nice mapgen:
viewtopic.php?f=9&t=11779 (I'll keep an eye on it)
AFAIK the closest subgame which goes into the direction what we want is BFG. But it is not very sophisticated, only a few new stone types, and they come in perfectly flat layers. It's a bit better than Minetest Game, but still not very interesting if you ask me.