Optics & optical coatings

The natural mineral is called fluorite and although it is rare, some fluoresce when irradiated with ultraviolet light. Artificial calcium fluoride is used for high-grade camera lenses or beamsplitters of spectroscopes.
It has optical properties such as high transmittance in a wide range from vacuum ultraviolet to infrared, and small dispersion of refractive index and different dispersion curve (abnormal partial dispersion) compared to average optical glass, making it very advantageous when designing achromatic lenses. Its isotropic crystal structure does not cause birefringence.
It is more generally used than lithium fluoride or magnesium fluoride due to its relative chemical stability and low deliquescence.

Sapphire is a hard mineral second to diamond, and is a crystal difficult to scratch. It has long been popular for use in watches as crystals, shafts of gears or bearings. Sometimes called sapphire glass, but its structure is not vitreous but crystalline.
Sapphire is chemically very stable making it useful as an alternative to glass. Artificial sapphire is transparent and colorless, and has a wide transmission range from ultraviolet to infrared. Since sapphire is a single crystal, birefringence occurs depending on the orientation of the crystal. While it has good insulation properties, its thermal conductivity is relatively high.
It is used as titanium-sapphire (Ti: sapphire) for an excitable medium of ultrashort pulse lasers or substrates that grow purple LED.

It is an amber color crystal, capable of transmitting up to long-wavelength infrared of 20µm. However, it does not transmit blue and green in the visible range.
Zinc selenide is often used as a lens material of CO2 lasers. Red laser light is sometimes used as the guide light of a CO2 laser, and zinc selenide can transmit this guide light together with the CO2 laser light. It is legally classified as a poisonous and deleterious substance, and some products require a document of transfer of poisonous and deleterious substances. In addition, disposal of used zinc selenide products as general waste is prohibited. Zinc selenide products no longer used must be returned to the place of purchase. It is insoluble in water, but reacts with acids to form toxic hydrogen selenide. Transmission loss by surface reflectance is high due to its high refractive index, but it can achieve 99% or higher transmittance with the addition of anti-reflection coating.

It is single crystal silicon used in semiconductors, has metallic luster and although it does not transmit light, it transmits infrared light from wavelengths of 2 to 6µm. It can also be used as a filter for infrared detectors.
Its excellent thermal conductivity also makes it useful as substrates of gold coating mirrors used for high-output CO2

This material has metallic luster and although it does not transmit light, it transmits a wide infrared range from 2 to 20µm. It is used as a material of thermography camera lenses.
Since it has a high refractive index of 4, its transmittance goes down to lower than 50% due to transmission loss by surface reflectance if used without an anti-reflection coating.

This material is a single crystal quartz without impurities (trigonal) and exhibits small birefringence.
Artificial crystals are mass produced as materials for low pass filters of crystal oscillators or CCD imaging devices.
Production of waveplates utilizes small refractive index difference between ordinary beam (no) and extraordinary beam (ne).

Calcite is transparent and colorless, and abundant in nature.
It is a single crystal (trigonal) and exhibits large birefringence.
Its refractive index difference between ordinary beam (no) and extraordinary beam (ne) is utilized in Glan-Thompson polarizers with high extinction ratio performance.
Calcite is a soft crystal and easily scratched.