Cleiophane

Cleiophane (sometimes misspelled as "clelophane" ) was a variety of sphalerite (also known as zincblende or blackjack) that often yielded large specimens of gemstone quality.

Description
Sphalerite was a mineral that occurred in crystals of yellow, orange, red, and brown. Cleiophane was sphalerite with pale green "flash" that made beautiful, faceted semi-precious stones, often in unusually large sizes up to 3 inches (8 centimeters) across. Unfortunately, cleiophane was not hard enough to be used in jewelry that was subject to shock or wear. A typical stone had a base value of 50 gp.

Powers
This gemstone had the remarkable ability to capture a three-dimensional mental image of the bearer and his or her surroundings out to a spherical radius of ten feet (three meters). The cleiophane gem had to be touched, worn, or carried by the subject when a spell of any type was cast by or on the subject. When these conditions were met, it was likely that a new image of that moment either was added to other images stored in the gem, or (more probably) replaced all previously stored images. This power was known as the "echo effect".

Any images stored in a cleiophane gem could be recalled by simply grasping the stone and willing them to appear. The images were silent, still, and faintly luminescent for three minutes. They appeared in the space next to the gem, filling a ten foot (three meter) sphere with a properly lit, exact reproduction of the scene in which they were created. There was no limit on how many times the image or images could be recalled, and no known limit on how long the images remained in the gem.