Diamond Spirit is a series of books about a girl named Jess and her horse.
From Amazon (Diamond Spirit 3 - Opal Dreaming):
Finally the day has come when Jess can bring home her filly Opal. But after Opal almost drowns in a flooded river, she falls ill and won't get better. Has Diamond's spirit abandoned her?
This is the final book in the Diamond Spirit series.
'What did you decide to call her?' asked Lawson, looking over Jess's shoulder.
'Opal,' said Jess, gazing lovingly at the little chestnut foal.
'Bad - luck stones,' grunted Lawson.
Finally the day has come when Jess can bring home her once-in-a-lifetime horse. But Opal falls ill, the local vet just can't find a cure and Lawson casts her out onto his cattle station to 'do or die.'
Wild with anger and refusing to give up, Jess heads west on an outback droving trip with her best friends - and Luke.
Will she be able to bring Opal home? And could Luke be her once-in-a-lifetime boy?