Difficulties with heat treatments of Tiger's Eye

Hi, I’m hoping to find an answer . . . I was given a strand of “Tiger
Eye” beads, approx. 8mm, but the customer wants them heat treated so
they turn redder.

I’ve put them in my electric fry pan (like I’ve done previously with
untreated Tiger’s Eye) and heated them to 500 F for as long as 45
minutes! They turn red during the heating, but return to the
original color when they cool off. What am I doing wrong? Could
these be synthetic?

Thank you in advance for any advice or suggestions!