I'm new here and I hope you'll like this theory
After reading the 937 chapter I think I've understand how the poneglyph have been created.
In this chapter Gyukimaru explain that Ryuma's sword became black and that's not how it used to be. This sentence reminded me of Mihawk's one telling zoro that his sword should also turn black if he wants to protect them.
With this I've thought that maybe if you're staying long enough with you're sword imbuing it haki in daily basic and therefore imbuing it your will it'll turn black. Becoming one of the strongest ever and no one would be able to leave a scratch ( chapter 779).A sword like that would be unbreakable.
Now I'll link that to the creation of poneglyph.
Wano is the homeland of poneglyph and of samouraïs.
Poneglyph are black, indestructible, have a will.
Road poneglyph are special since there're only 4 and you could still think that the creation is the same but still having blood mix within to stamp the difference.
The creation was done by the kozuki who are obviously strong enough warriors to use haki efficiently.
Oden knew how to create them, and the secrets of creation should have been passed to momo, so the lack of materials is not the issue here.
Robin is often seen touching poneglyph without being weakened by it, so I don't think it's kairoseki
And there is definitely a will in the poneglyph, since Roger used the voice of all things to hear them, which I think now is directly link to haki.
In conclusion I think the poneglyph are indestructible not because of the stone or iron used to make them but with the haki imbued in them still there after the death of the makers. And that is why Roger could understand them.
I hope you've like my first theory and sorry for my bad English