Magnesium Fluoride is used for optical elements in the infrared where extreme ruggedness and durability is required. Its useful transmission range is 0.11 µm - 7.5 µm. Irradiation does not lead to color centers. Magnesium fluoride is a rugged, hard material which is resistant to thermal and mechanical shock. Considerable mechanical shock is needed to cause cleavage which is near perfect when it occurs. The natural form of MgF2 is known as Sellaite. Magnesium fluoride is a positive birefringent crystal grown normally to 135 mm diameter by vacuum Stockbarger technique, seeding along the C-axis.