Have you ever noticed that Ruby has a hot tempered and fiery personality in comparison to Sapphire's calm and collected one? I have a theory that may answer the reason behind this, in the Gem Army; any of the Rubies that volunteered to part of the Homeworld Army were harshly trained by the Gem commanders and they were told by their superiors "that the only temper that you should have on the battlefield is a fiery one."

These Rubies acted as the grunts of the Gem army and were probably the first troops sent onto the battlefield in front of any of the other soldiers, so they would probably be the first ones to be poofed as they were probably the weakest in the eyes of any of the Diamonds.