Magnesium Fluoride (MgF2 crystal) is used for optical elements where extreme ruggedness and durability is required. MgF2 Crystal's useful transmission range is 0.11 um - 8.5 um. Irradiation does not lead to color centers. Magnesium Flouride is a rugged, hard material which is resistant to thermal and mechanical shock and can be worked to the highest standards. Considerable mechanical shock is needed to cause cleavage which is near perfect when it occurs. MgF2 single crystals is a positive crystal and slightly birefringent and usually supplied with the optic axis cut perpendicular or 45-deg to the window faces.