|When tiger eye is blue it may also be called "hawks eye". All of these stones cut by the author. The first two stones are blue to blue-green and the third stone is a composite with black jade. The tiger eye in the third stone is a variegated hawks-eye / tiger-eye.
The blue coloration is due to the preservation of the original amphibole, riebeckite (the fibrous material enclosed in the chalcedony.) The name "crocidolite" is actually the fibrous variety of riebeckite. The yellow coloration often found in tiger eye is cause when some or all of the iron in the original amphibole begins to turn to limonite (yellow-brown) in color.
It is now thought that hawks eye contains the original riebeckite encapsulated in clear chalcedony.