I am not sure which detector model you have, but the three Owner's manual clips I am including here ought to answer your question. they are from the Teknetics T2 and Omega, and the Fisher Gold Bug Pro (which should duplicate the Teknetics G2).
Teknetics T2:
BAR GRAPHS Fe3O4 (magnetite)
This bar graph displays the magnetic mineralization factor, or magnetic susceptibility, of the soil. Magnetic susceptibility is expressed in terms of the percent volume of the iron mineral magnetite, which most black sand is made of. The depth to which objects can be accurately identified is strongly influenced by the magnetic susceptibility of the soil. High Fe3O4 values have a greater effect on detection depth in the Discrimination mode than in the All Metal mode. For the most accurate Fe3O4 reading, pump the search coil as though you were ground canceling.
Fe3O4 approx.
Range micro-cgs Description
3 ... 7,500 ..... uncommon but not rare, heavy mineralization
1 ... 2,500 ..... heavy mineralization, not uncommon in goldfields
0.3 ... 750 ..... heavy mineralization, but not uncommon in some regions
0.1 ... 250 ..... medium mineralization, typical
0.03 ... 75 ..... light mineralization, but common
0.01 ... 25 ..... light mineralization, often low G.C. setting
blank <14 ..... quartz & coral white beach sands
Teknetics Omega:
Fe3O4 BARGRAPH
The Fe3O4 4-segment bar graph indicates the amount of ground mineralization, independent of type, expressed as an equivalent volume concentration of magnetite (Fe3O4). It updates every second. It is sensitive to motion and will give the most accurate readings if you