Charge counters are indistinguishable from one another. If some other spell or ability (such as Power Conduit’s ability) puts a charge counter on Jeweled Amulet, activating Jeweled Amulet’s second ability will still check the type of mana last noted by its first ability. If there is no type of mana noted by its first ability (because it hasn’t been activated by that point), the second ability will produce no mana.