Whether its natural or man-made, diamond solves some very demanding technical requirements.  Natural diamond is used as a window material in extreme pressure applications.  One such application is microscopy where the diamond is the base against which the sample is compressed.  In this type of use, the diamond will withstand the compressive stress and abrasion as the sample is manipulated while also allowing the sample to be back-lit.  Natural diamond is also used for fiberoptic process probe windows when monitoring, or analyzing, very abrasive materials.