The best I can figure is someone went through a random collection of scenes never used for other books because they weren't very good, shuffled them into a chronological order, and then typed it up with consistent names.
It's a mess, and none of the aspects rise above thoroughly mediocre: half-hearted Gothic, suspense, romance, travel, adventure, wish-fulfillment, etc. And a really surprising number of bastards or children who were legitimized by marriages between their mothers and people who were not their fathers.
Disappointingly, the Black Opal of the title is pure McGuffin, everyone ends up well off in a lovely home, the three possible love interests don't seem to interest the heroine much, and events are too random to even be coincidental. Of all the squares I considered using it for, it didn't really live up to any of them. I'm going with Gothic because it does have recognizable Gothic elements, even if they're not well-developed.
Nonetheless, it was an interesting read. It wasn't like the Victoria Holt books I read in the 70s, nor is it at all like contemporary romance or suspense. Although it lacked a real commitment to formula, it was very definitely written by someone who knew what would make an enjoyable read. Consider it a lesser work by a real pro. It certainly didn't put me off Holt: I have a couple more I'm considering.