Supposed to be a silver coin I believe. Also supposedly dated the year she was built. The coin is so you will have the dosh to pay Charon to ferry you across the river Styx should you and your ship meet an unpleasant fate.
Whether or not you can use Amex and claim frequent flyer points is open to debate.
"Do you think God gets stoned? I think so... Look at the platypus." Robin Williams.