A method and apparatus for characterizing objects. The method includes the steps of illuminating the object with incident red light having at least some wavelengths between 620 nms and 650 nms and detecting red light fluorescence from said object having a wavelength greater than visible wavelengths greater than that of the incident wavelengths, for example by using a filter. An apparatus including a source of red incident light, a detector for longer wavelength fluorescent light and a means for physically removing the detected objects from the rest is also provided. An embodiment of the present invention may be used in a mine, for example, to separate gem stones from less valuable ore rock or in prospecting to detect the presence of gems. In this embodiment the detection is not possible with the naked eye alone.