I said Calcium Lime and Rust. There is also Lime-Away.
She put the CLR in there. Then she washed it out many times. All of the mineral deposits washed away. She rinsed it out with water many more times, and then eventually cleaned it with vinegar after rinsing it many times. At least, I think that's how she cleaned it. She's the kettle cleaner, I don't know much about it, other than that CLR exists.
Ah, didn't realize it's the kind that has the element exposed. Those aren't common where I am. The cheap kettle I have uses a hot plate instead so I forgot about the other variety. Don't blame you for going back to the stovetop one then