Zinc Selenide etalons  have a reasonably high index (~2.5 depending on wavelength) that creates a reasonably high finesse without any coatings.  They also remain transmissive over a very broad wavelength range from 600nm to 20ums!.  Thermal tuning is high but not as rapid as silicon or germanium.  Unfortunately zinc selenide is one of the few optical materials that we cannot fluid jet polish (due to the grain structure), so we cannot make the parallelism super uniform and coat them to create high finesse etalons.  But for low finesse (~1.6) etalons that work over a massive wavelength range, they are pretty handy!  

• Material: Zinc Selenide
• Wavelength range: red to Long IR 0.6um to 20um
• Wedge < 2.5 arc second
• Clear aperture: 8mm
• Surface quality: 40/20 or better
• Transmitted wavefront error: λ/8
• Diameter: 12.7mm
• Finesse and FSR are dependent on the wavelength