This sounds like Shadow Magic, which was the first book published by Patricia Wrede (also author of Mairelon the Magician).
As it says in this review, the main character is a young woman named Alethia. There is a war brewing with the Shadow-born (shadowy evil creatures which were bound long ago) and their agents the Lithmern, and she gets kidnapped. She ends up takes refuge in the Shee city. (The Shee are sort of like elves -- slightly pointed ears, have magic, etc.) Partly due to a prophecy, and partly due to some inadvertent magic she did on her own, they start teaching her magic. The focus she chooses ends up being a "firestone", which are "natural amplifiers of power".
Later, when she leaves the Shee city to return to her family, her group is attacked and a storm comes out of nowhere. She is separated from everyone, and she uses her magic to find shelter. The next morning...
she glanced out of the cave mouth. And froze. The sky was no longer
simply heavy with clouds. To her newly awakened spell-sight, it was
shot with dark lightenings. Flashes of blackness leapt along a web of
power that could only have been constructed by the Shadown-born.
As she began to comprehend more fully what she was seeing, Alethia
relaxed. She had, she realized, a tremendous advantage over the Shee
wizards. As long as the spell-sight was working she would instantly
know of the use of magic anywhere around her, without having to resort
to detection spells which might betray her own presence.
As she is using the spell-sight in the cave, she realizes that there is a "golden glow of raw power" from the back of the cave, and it turns out to be a magic artifact that had been missing for years.
There was no doubt in her mind that she held the long-lost Crown of
Alkyra; nothing else could possibly hold such power.
There are several other books set in the same universe -- "Lyra". Other non-humans in these books are the Wyrds -- smaller sort of "tree elves", and the Neira -- sort of like mer-people.
