[1] This is a Faraday rotator material with excellent transparency properties and is very resistant to laser damage.
TGG has a high Verdet constant which results in the Faraday effect.
The Verdet constant increases substantially as the mineral approaches cryogenic temperatures.
[2] The highest Verdet constants are found in terbium doped dense flint glasses or in crystals of TGG.
Many Faraday rotators and isolators are adjustable by varying the degree to which the amount of the Faraday rotator material is inserted into the magnetic field of the device.