"The markings on the sides of the stone indicate that it was once set in something."
It can be dropped by the Novistadores, and is the rarest eye to find. It is possible to get one guaranteed blue eye (other than this, you can go through the game without even finding another one): there is a hive on the ceiling of the dance hall in Chapter 4-1, right before/after Ashley is kidnapped by a Novistador. Shooting this with enough firepower will trigger a scene and result in many eyes dropping, one of which is blue eye.
One of the ways to farm this item is to save the game right at the beginning of Chapter 3-2, where Leon enters the waterway, and kill the Novistadores until one of them drops a blue eye. If you are unlucky, reload the game and try again.
In Separate Ways, there is only one, and it is found inside a barrel in the first area of Chapter 5, near a Gatling Man.
- Hamamura, Koichi, ed (2006) (in Japanese). biohazard4 kaitaishinsho revised edition. Famitsu. ISBN 978-4-7577-2638-3.