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Foundation 4 of 12
Revelation 21:19

Emerald

σμάραγδος

The pivotal stone: where the breastplate had garnet (which cannot split light), Revelation has emerald, the swap that makes all twelve stones split light.

Mineralogy
Greek name
smaragdos (σμάραγδος)
Modern identification
Beryl (var. Emerald)
Chemical formula
Be₃Al₂(SiO₃)₆
Crystal system
Hexagonal
Optical class
Uniaxial (−)
Birefringence (Δn)
0.004–0.010
Refractive indices
1.565–1.602
Color / appearance
Green (Cr/V-bearing)

Strong dichroism: green/blue-green. The one substitution from the Exodus breastplate that changes the list's optics (replacing cubic, isotropic garnet), completing the list as wholly anisotropic.

The light between

Emerald is anisotropic: enter it with a single ray of light and it leaves as two, split apart by the crystal's inner order. This is birefringence, the same property shared by every one of the twelve foundation stones, and by none of the cubic stones left out of the list.

In the high-priestly breastplate of Exodus 28, this position held Carbuncle (Garnet?), a cubic stone that cannot split light. Its replacement here by Emerald is the single substitution that makes all twelve stones light-splitters.

Hold the light in your hand

The Revelation Stones collection brings each foundation stone into the room with you, and the Light Between research explains what you are looking at.