http://us.battle.net/d3/en/forum/topic/19876687565

You use up 3 Flawless Royal Gems and a Legendary Gem to gain the Flawless Royal Gems stat to any Ancient Item! Read the linked guide above.

You could potentially put an extra 750 main stat per item and do massive damage.

FYI: I'm sure you could purchase high level Legendary Gems from:

http://forums.d2jsp.org/user.php?i=832407
or
http://forums.d2jsp.org/user.php?i=784037
or
other people, or just get them yourself!

Good luck and happy damage boosting!