 The loupe (hand lens) is a vital geological field tool used to identify small mineral crystals and structures in rocks.Loupes are used by professional and amateur field biologists for help identifying species in field situations where a full-sized microscope is impractical, but the ability to observe small morphological characteristics is desired.Many floras and diagnostic keys for identifying plant or animal species recommend the use of a loupe, because taxa may be separated by minute details like the presence of hairs, shapes of hairs and glands.