How durable are the infinity gems?

Could someone like Hulk smash one into pieces...?

Nah probably not

No he can't as far as we know. It took a multiversal collapse to shatter them. Not sure if there were any other incidents of the gems being broken, but I doubt it.

The gems were originally intended to be indestructible as shown in Avengers VA Ultraforce.

During Bendis's run of the New Avengers Reed collected a complete infinity gauntlet and tries to will the gems out of existence but it did not work because they were meant to be eternal.

However things seem to have changed once Hickman came on board with New Avengers.

Still probably would be too durable for the likes if Hulk even if it is no longer completely indestructible.

Currently all but the Time Gem have been destroyed. During an Incursion Event Captain America wielding the Gauntlet pushed the other Earth back into its own Universe and stopped the Incursion. I suspect there alleged destruction was caused since two universes where overlapping and the gems powers while supreme do not function outside of their native Universe.

Ask Goku.

Goku ain't need no gems. He got dem Dragon Balls.

Which is more powerful? What does the infinity gauntlet even do?

Batman.