A friend of mine in our campaign recently got a masterwork mundane-crystal great axe after his last great axe was eaten by a rust monster. The DM said that since its made of crystal its more likely to break and if he uses it against a heavily armored target or something made of stone it would have to roll to see if it shatters. But from what I'm reading on it, it seems that its less likely to break than a steel great axe. I'm fairly new to D&D so i just wanted to get an outside opinion.
The fortified crystal possesses the properties of a similar masterwork steel weapon or armor
Unless your DM is using a houserule to incorporate steel weapon wear and tear, crystal weapons shouldn’t break either. D&D 3.5e does not include any rules by which a weapon breaks from you using the weapon (only rules for it being damaged when other things specifically attack the weapon, as with sunder).
The crystal is considered identical to steel as far as most of its material properties are concerned, and is even considered metal for the purposes of druids’ limitations. And indeed, many real-life crystals are quite sturdy, and not easily broken—for that matter, steel is itself crystalline.
The crystal is slightly easier to break when you’re actually trying to, though: it has hardness 8 (to steel’s 10), and 25 hp per inch of thickness (to steel’s 30). So if someone actively tries to sunder this great axe, it will be slightly easier than it would be if the great axe were made from steel. This is the only difference between steel and mundane crystal. Note that each +1 of magical enhancement on the item adds +2 hardness and +10 hp to the item, which may quickly drown out the slight differences between the materials.
Please note that wear and tear rules work poorly in D&D 3.5e; personally, I have never seen a well-made one. Mundane gear maintenance is an assumed part of characters’ background activity, and outside of explicit attempts to damage items (whether it be through sundering, or effects like the rust monster you ran into), gear does not break as part of the game. Since gear is absolutely crucial to many characters, changing this is a big change, and introduces a drastic vulnerability for those characters. Since those characters are simultaneously those that are weakest anyway, this rarely works out for the best in a game.
For what it’s worth, simply put, if a game were advertised as including such rules, I would decline to participate. If such rules were sprung on me in the middle of a game, I would push back, firmly but politely. If the DM remained adamant about keeping them, I would leave that game. It indicates a number of things about the game that do not fit my tastes.