People emigrated to this planet from Earth through a rift that is now closed. Remnants of older alien civilization, including an amber city that is dangerous to humans. Some amber colors have powers, and some people can "use" amber. There are different heroines in each book, and often a romance with a Ranger.
Obsidian Prey (Ghost Hunters, Book 6), August 25, 2009, by Jayne Castle?
The world of Harmony has its own special glow, thanks to the aliens who left behind powerful amber and twisting underground catacombs. Sparks fly, senses are rezzed, and passion can take you by surprise...
This is one book in a lengthy series.
Google search: science fiction book earth "harmony" "amber"
This isn't a 100% match, but it might be a book from Jane Fletcher's Celaeno series. I certainly think it's worth taking a look.
There are different heroines in each book
Ellen Mittal in Book 1, Chip Coppelli and maybe Katryn Nagata in Book 2, Lynn in Book 3, Natasha Ionadis in Book 4, Riki Sadiq in Book 5.
and often a romance with a Ranger.
Chip and Riki are both Rangers, Lynn has a romance with a Ranger called Kim, I think Chip has a romance with one of her fellow Rangers too.
People emigrated to this planet from Earth through a rift that is now closed.
They had emigrated, but I am pretty sure they used a spaceship, not a rift. That said, I don't think they can return, and technology and learning have regressed too far for them to be able to use the ship again even if it's still functional.
However... I have been looking through Goodreads and I can't find anything about an older civilisation. And if magic amber is involved, it isn't prominent enough to be mentioned by the Goodreads reviewers.