Well, Flints are one of the most basic and unique elements of clonk - I can't come up with something more interesting that might also be found as a natural occurence in the earth. Yet we might consider removing all but the normal flint and add more constructable explosives like bombs and dynamite with other functions than exploding on contact - of course, this would alter fighting in Clonk a lot, but might be just one of the gameplay changes we try to archieve ;)

Edit:
I can't make the texture exactly seamless, because the uv-mesh has an rather obvious seam direktly on the "back" of the flint which doesn't fit to the edges of the texture.

No, you can't explain flints away with volcanoes. No flint survives a volcano. They also wouldn't survive any other plausible history of the landmass, so we should just embrace the silliness and admit that flints are there because explosions are cool. On that thought, how about glowing flints? To emphasize the magic that they are?

Imo, it's more important that objects look *different* than whether they look correct by some weird definition.

Keeping that in mind: Objects should have more characteristic shapes. For example, rocks could have random, sharp-edged shapes. Flints could be round. Loam could come in square cubes. Concerning loam, we could also abolish it and allow construction of bridges just using earth.

Flints are an effective method to provide the player with basic explosives at the beginning of the game. Otherwise we need another technique for the player to gain the first resources (a pick-axe?).
The only other kind of explosives I can imagine right now are some kind of a gunpowder grenade (http://upload.wikimedia.org/wikipedia/de/thumb/e/ef/Old-granate.jpg/180px-Old-granate.jpg) which would need a lot of materials to be plausible (gunpowder + metal, gunpowder from sulphur or in a pseudo realistic way sulphur + coal).
These grenades could replace the normal flint by exploding on contact (then we have replaced a red ball with a black ball with the difference that the black one is harder to acquire) or the T-Flint with delay (that does not solve the problem cause I think we do need anything like the basic flint).
Why do I think we need anything alike the flint? Mainly for the mining reason. We may abandon the flint if we implement something like the pick-axe (or any kind of easy to access explosive). The clonk to clonk combat will be improved by the weaponry everyone plans to contribute, the combat against buildings may be fought with refined explosives (I even thought that the rocks may damage building when fast enough like shot from a cannon).
Anyway, I think the basic flint is versatile and funny enough and solves a lot of problems. The better flints are another story. For these are mostly used for destroying buildings or advanced mining it's possible to remove them. In that case the above-mentioned grenade would fit (with a little delay and a sticky function?). That would provide us with the opportunity to provide the flint a more pleasant look cause there are no more several red balls the player needs to distinguish between. The flint could get some yellow veins or so. And I think we can save us any explanation why the flints are there and just say that they are there... ;)

I think the flint should have a higher recognition value - not looking like some lump of lava.
I think we should keep the dark red - maybe with a bit contrast using bright yellow stripes/dots/pattern

Looks cool! The texture is imo a little bit too lava-lumpy as Zapper said. In any case, can you post the model with the texture here?

I like this one very much. Perhaps more harder displacement?