I'm just looking at the poll she's put up with what the lyrics are in IKIHAH. I'm out of step with a lot of her followers! 2/3 seem to think it's "heart of stone", some kind of Six crossover wishful thinking?
And I can reveal, with my geological background, that if she's "made of steel, hard as stone" the stone she is referring to is probably orthoclase feldspar (you heard it here first!).
(With the caveat that that's if the stone is hard as steel. Stones can be very hard (diamond) or very soft (talc).)