No worries, we never intended to make any armor type useless. Particularly the leather! It will be a must to have, if you want to step inside the cave to get the bronze. Armors will go hand in hand with metal age player is currently in.
And yea, there will be no diamond armors, however, various gem decoration (on your armor piece) for visual purposes is certainly thinkable! I would like using diamonds for encrusting drills, few high tier tools, jewellery, as an expensive trade good and perhaps to simply hord them under a beautiful display case to show off your wealth.