An apparatus for determining the nature of the formation at the bottom of a wellbore being drilled with a tubular drill string through which drilling fluid is pumped to return the drilling fluid to the surface via the wellbore, the apparatus comprising a tool body for installation in a drill string with a bore extending there through and an ultraviolet fluorescence detector in a passageway in the tool body, the tool body being configured to receive drilling fluid from inside the drill string and to receive drilling fluid from outside the drill string, wherein the drilling fluid can be directed past the detector so as to enable measurements to be made.