Elemental versions will definitely be more powerful than the non-elemental versions, and will be rarer, and have rarer drops. Exactly how the balance will fall I'm not sure, but I think an elemental tier 3 could be more powerful than a tier 4. It won't just be straight forward though as different elements will change more stats than just damage and health.Toxin creatures may do damage over time for example, and shock creatures may move faster.
The rarer material types for weapons will be generally better than the less rare ones, so Diamond will be better than Copper which will be better than Stone; however, it won't be a completely straightforward, as there will be a lot of other variation between them too. So Iron weapons might have high durability, high damage, but be very slow, whereas Copper would have moderate durability, moderate damage, and be quick. You may decide to use the Copper over the Iron even though Iron is slightly rarer. This will be particularly prominent with the gem weapons as we will balance so that they are all of equivalent power, but each feels very different to use.
On top of the base weapon there are three systems that can adjust its stats: the crystal that you put into it (munitions), using the forge to upgrade it, and your character's stats.
The resulting stats of a weapon which has a crystal loaded will be a combination of the base weapon and the crystal, so a crystal will modify the stats of the weapon rather than replace them, and may add other effects too. You can put any crystal in to any weapon, but the stat modification will be such that putting a really powerful crystal into a very weak weapon will make it somewhat better, but the the law of diminishing returns will apply so it will probably be a waste of resources / coin.
Overall the aim of these systems is to create a real sandbox around the stats of weapons, tools and equipment, which along with the character progression system allows for players to create their own combinations, and have their loadouts play the way they want them to play.