Not much is known about Emerald. She's not a legendary figure in the same vein as Alyce Ambrosia or Leith Torles were, but very likely she also played a vital role at some point. After Ruby finishes conquering the sixth of The Seven Wonders, she sees not only the adventures of Alyce and Leith with Iskandar, but also with Emerald at the Nakle Lines.
One particular point of note is her name. Sapphire and Ruby both are named after gems, so it seems some kind of connection might be implied. It is probable that the sisters may be her descendants.