It is usefull tool for people who pick up all Flawless Square Gems. You choose quantity and type of gems and you get clear result page with information like which high tier gems you can make, how much it cost and how many Tome of Secrets/Demonic Essence you need to craft them.
Ofcourse everything is online, no need download anything and it is 100% legit with Blizzard Rules. My site uses very simple Blizzard API for showing items in tooltip.
Looks very good! It'll help for people debating between buying lower tier gems/tomes to craft or buying high level gems straight off the AH. I've always wondered how much gold could potentially be saved that way.