The color Black is for Isis Mourning
Red is also her color

Lady in Red, Lady of the Red Apparel, and Black-robed Isis
So the Black and Red inner four stones go with the Isis theme
She is Mistress of all the element
Queen of the Winds
Queen of the Earth
Queen of the Rivers
Lady of the Flame

The Greco-Roman festival of Ploiaphesia, also called Isidis Navigum, the Festival of the Ship of Isis, celebrates Isis as sea goddess and as the goddess of navigation