Can't remember the title of a fantasy novel

[Mooncalf1978]

The title and author are completely escaping me - must be age kicking in! Please help.

I'm afraid my memory is fairly sketchy but I am quite confident that the main protagonist was a girl. Maybe a slave girl - not sure about that though. I a, sure that she had some sort of power that she got from stones or runes. When she had these stones\runes she could create light and maybe heat?!
I believe she may have been taken from her village and transported by boat where she was guarded/protected by some sea-type creature similar to the niskies from Memory, Sorrow and Thorn. My only other memory is....spoiler....towards the end she is imprisoned in a sea cave and left for the tide to rise and drown her.

Anyone have any ideas? Please help!!!!

 

[bangerm]

sounds like something i read by andre norton...some "witch book" or moonstone type of thing
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[digs]

The slave-girl who has an affinity with stones reminded me of Threshold by Sara Douglass (although in that the main character has a connection with glass), but it doesn't fit with the sea creature thing.