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Foundation 2 of 12
Revelation 21:19

Sapphire

σάπφειρος

Dichroic: it shows two different colors depending on the direction light travels through it.

Mineralogy
Greek name
sappheiros (σάπφειρος)
Modern identification
Corundum
Chemical formula
Al₂O₃
Crystal system
Trigonal
Optical class
Uniaxial (−)
Birefringence (Δn)
0.008–0.009
Refractive indices
1.762–1.770
Color / appearance
Blue (and other colors)

Strong pleochroism; dichroic. Corundum crystallises in the trigonal subsystem (space group R̄c).

The light between

Sapphire 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 Topaz in the Septuagint reckoning.

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.