Tressed with redolent ebony.
Writing of the mermaid, the lines are more suggestive of the shell nacre:
Combing her hair
Under the sea,
In a golden curl
With a comb of pearl.
Again in a sonnet, he evidently refers to mother-of-pearl when he says:
All night through archways of the bridgèd pearl,
And portals of pure silver, walks the moon.