Galium selenide (GaSe) is hexagonal crystal mostly used for second harmonic generation (SHG) in far infrared (FIR) wavelengths. Gallium selenide has extremely low Mohs hardness, which makes it very fragile and eliminates possibility to apply mechanical treatment or coatings. 3photon could offer GaSe crystals cut along optical axis Z with cleaved surfaces.

https://3photon.com/gase-gallium-selenide/