Due to its unique proporties, black granite has in recent years been used extensively in the field of measuring instruments, both for traditional items such as surface plates, rules, cubes and prisms as well as for three dimensional measuring machines. Suitably lapped black granite surfaces are not only extremely precise but are also ideal for use with air bearings.   The reasons for using black granite in constructing precision units are as follows:   - Dimensional Stability; black granite is a natural aged material formed over millions of years and therefore offers great internal stability.   - Thermal Stability; the coefficient of linear expansion is much lower than for steel or cast iron.   - Hardness; comparable to good quality tempered steel.   - Durability; instrument lasts longer.   - Resistant; to oxidisation and acids, no corrosion, no maintenance, non magnetic.     Black Granite Surface Plates Black Granite Supports Black Granite Gauge Stand Supports Rotating Table on Air Bearings Air Bearings PERTEST - Perpendicular Test Instrument Black Granite Accessories   Black Granite - Special Working