Another possibility for imitating sapphire (blue corundum) is blue quartz, which gets its coloration from inclusions of crocidolite or rutile fibers. Yet another possibilty is dumortierite quartz. The latter tends to be opaque, while blue quartz is somewhat translucent (compare with aventurine). I haven't seen enough dumortierite quartz on the market to judge its qualities. I have seen some rather nice specimens of blue quartz being sold on the internet. Both matierials have a Mohs hardness of '7', making them durable enough for any jewelry application.