diamond
Asami knew her precious jewels. She had a rather expansive collection of jewelry that would probably pay for an entire neighborhoods power for several months. She couldn't deny. She had a weakness.
It had started with her mother really. Her mother was a beautiful woman. So beautiful, in fact, that Asami had often been envious of her beauty. Her mother could wear the most beautiful jewels and make them look like mere trinkets. She would often let Asami wear her jewelry but Asami always felt overpowered by the jewels. Like she didn't shine quite like her mother did.
After her mother died, Asami inherited her mother's collection. She wore each of them interchangeably and she became quite fond of them not only because they were beautiful but because they reminded her of her mother. Hiroshi bought her many jewels birthday to birthday, but none were quite as spectacular as her mothers.
Except Korra's jewels.
Korra had a knack, being a quite superb earth bender, for crushing stones so much they rapidly formed quite spectacular jewel specimens. It was her earth bending abilities coupled with fire bending that enabled her to do this, though Korra had never thought much of it. Until she had someone to make jewels for.
It started with innocuous gifts. Tiny emeralds to match Asami's eyes. But Asami wore them so often that Korra got it into her head that these jewels would be an excellent scapegoat for when she did things wrong.
Like crashing Asami's favorite car.
That day she'd presented a diamond to the heiress so big that it might've blinded the whole city if it was let to sit in sunlight.
Of course, Asami was so impressed with the gesture that she didn't find out about the car until much later that night. If she'd been a fire bender she might've turned the whole world to cinder.