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Blue Tiger's-eye

If I remember correctly from my tiger eye cutting days in the 80's,
blue tiger eye is a heat treated form of golden tiger-eye. 

Dave, the material called blue tiger’s-eye occurs naturally in shades
from blue-gray, blue to blue-green. I’ve also cut beautiful
variegated pieces in blue-brown and green-brown and some displaying
all those colors as well as pink in the same stone. I’d always heard
the blue material called Hawk’s-Eye and was surprised to learn, when
I looked it up in Schumann to refresh my memory, that he classifies
it separately. He says tiger’s-eye is formed from hawk’s-eye after
iron in the crocidolite inclusions oxidizes to brown. I know from
personal experience that when brown tiger’s-eye is heated carefully
in an oven it turns red.

Some fine-grained brown tiger’s-eye can be acid-bleached to yield
pale honey-colored stones. These are usually cut in Germany and sold
as cheap cat’s-eye substitutes. Of course true cat’s-eye is
chrysoberyl although cat’s-eye-effect stones can be cut from many
minerals: scapolite, tourmaline, beryl, feldspar, some garnets,
prehnite, alexandrite (also chrysoberyl) and lots of others. I even
cut some small ones recently from a parcel of Madagascar neon apatite.