If even a group of first-level adventurers can get their hands on your object, they can throw it off a 200ft cliff, dealing it 20d6 of physical damage. Checking the damage to objects page, it looks like physical damage to objects does not get halved, so this will deal your object 70 damage on average and 120 damage at most.
Low-level adventurers can get high damage output from Spirited Charge. A third-level character might have strength 20, bulls strength, enlarge person, and barbarian rage, giving 3*(2d6+15) lance damage which is 51 damage at minimum.
The highest hardness you can reasonably get is around 35 (using an adamantine object and a caster-level-30 hardening spell). The conclusion we reach is that, if adventurers get their hands on your object, they can destroy it.
Browsing the permanency spell options, we notice that you can make prismatic sphere permanent on an object. You can then use polymorph any object to make your object have a radius of 9.999 feet, so nobody can teleport through the sphere to get at your object. This will prevent adventurers from running off with your object unless they have the spells to disassemble the sphere, including disintegrate which requires an eleventh-level caster (or scroll).
If a group of seventeenth level adventurers gets their hands on your item, they can just cast mage's disjunction on it. This shuts down your prismatic sphere and your hardness spell.
I don't see a way through this. As far as I can tell, if the adventurers can get their hands on your object, they can destroy it.
Let's talk about -- well, not hiding your object, but at least making it difficult to retrieve. I propose making your object out of diamond, casting hardening and animate object, and throwing it into a volcano with instructions to burrow as deep as it can get. The "throw it into a gas giant" (or, better, the sun) ideas from the Hide Your Magic Item page also seem promising.