Hebrew-Mormon Seer Stones

Seer2
Described as flat crystals or transparent gems, the High Priest of the Hebrew Temple would hold the Urim and Thummim over his eyes like spectacles - one red and one green - to help induce religious visions (see link in comments). In this way, the Urim and Thummim would force visual processing of colors to occur as a stereo blending between the eyes rather than for each eye separately prior to reaching the visual cortex where colors are recognized.

For instance, by separating the brain's signal processing of red and green through the red-green cones of each eye's retina, the color yellow is synthesized in the visual cortex as a stereoscopic function. When combined with entheogens, psychoacoustics, symbolisms and meditation ritual, this stereoscopic stimulation of the visual cortex might have assisted in synthesizing visual constructs.

It is a well-known feature of psychedelics to envision red and green (or pink and chartruse) colors superimposed with patterning in the visual field. Scientific studies of psychedelics have confirmed this:

"Viewing the faces of other people in the room, the subject observed red and green glowing lines crossing their faces horizontally but following the contours of the face."

and

"Whenever the subject closed his eyes, he was presented with a dazzling array of colours and patterns. Most of the patterns were of a repetitive nature, resembling mandalas. The predominant colours were red and green and the predominant patterns were formed from circles and octagons."

Given this, perhaps the Urim and Thummim were used by the priests to separate pattern layers during hallucination in much the same way as 3D anaglyphic glasses do in 3D movies. They might have helped pull recognizable shapes out of the background noise, making it easier for the brain to recognize subtle visual cues synthesized by...or received by...the brain...from somewhere...

* Note: the spectacles in the photo were used by Nicolas Cage in the movie National Treasure.