The way I have conceptualized them is that they usually exist only as hilts, but when they are drawn from the scabbard, the blade comes into existence. The scabbard isn't really a scabbard at all, just a little thing that holds the few inches of blade that's always there in place. When they are first found by the characters in the book, they have no pommel stones. They are found later. One is a sapphire, one is an emerald. The stones give the blades more power/virtue/whatever and also (and here's the major plot spoiler) a means of unification. When the stones are pushed together, they become one, a completely clear diamond, and the swords become one two-bladed sword (think Darth Maul here). Possibly, there's some flame involved, either before or after unification. Have you read the first bits of the book I've linked to? If so, you might catch some foreshadowing...what else is blue and green? :)
Spencer |