OMG stands for "Oh my God", just to show her shock at a Melo pearl. I kinda feel sorry for you jumping into this.
Some of the experts on here should be around soon to offer some advice on value. I know Jeremy Shepherd sells exoctic natural pearls, so he may be helpful. Personally, I thought it was a conch at first, but seeing how round it is I would quess Valeria's opinion is more likely correct. It is very lovely, and good luck on your search.
Ooops, just re-read your post, I guess you already know the OMG meaning. Yes, natural pearls are very rare, but I don't know specifically about the melo.