Amber: Something Wicked
An Immortal, if he/she knows the technique, can turn himself into a nigh indestructible red gem which contains their personality, soul and power. It’s a last ditch survival measure.
There are techniques, including spells for turning Immortals into soul gems against their will. This Happened to Big Red. Depending on the technique and how precisely it was employed you can get anything from nearly all power and minimal soul to the opposite.