I am but a mere geo who can detect his gold wedding ring at as great a depth as he wants to dig, and I have never worried about the details of why. But I don't understand this discussion at all (seems to me that a person who has an average detector and spends as much time in the bush detecting as I see people spending on detector discussions here, is likely to be the most successful - barring issues like having a poor detector for noisy ground). I know little about the working of detectors and am amazed at the complexity of what people discuss and the claims they make for things that don't seem to me to vary a lot in overall complexity (except relating to search area of the coil, induced field strength from the coil, and noisy ground), My father made me my first detector around 1970 (wound the coil and all - he had his own electronics company).
In my naivety I have always considered that the property that mattered was conductivity (i.e. inverse of resistance). The basic principle seems to me to be that you are inducing an electromagnetic field into the ground via a coil. If there are no conductors in the ground there is no current flow induced in the ground as you move the coil. However if there is a conductor such as a gold nugget in the ground, the fact that a conductive lump is being subjected to a moving electromagnetic field will induce current flow in the conductive lump. That current flow will produce a secondary electromagnetic field around the lump, proportional to the size of the lump, and this will add to the field from the coil and give a different field to that induced by the coil alone. This would presumably be reflected by measuring this as the change in horizontal or vertical component of the field, and this change can be measured as an EMF which can be used to make a noise or drive a meter. Isn't this the principle in its most basic form? While mass is important I would expect shape to be a significant but lesser factor than mass, since it determines the orientation of the induced electromagnetic field and therefore the magnitude of the measured vertical or horizontal component of that field (the field around a vertical sheet would be quite different to that around a horizontal sheet - but without a conductive metal there would be no field anyway).
If so, conductivity (and mass and orientation of the lump) is the thing that matters, not SG. Conductivity is commonly measured relative to the IACS (the abbreviation for International Annealed Copper Standard). The number preceding "IACS" is the percentage of conductivity a material has relative to copper, which is considered to be 100% conductive.
Silver (the only metal likely to be in a gold nugget other than gold) is slightly more conductive than copper (105%). Gold is not nearly as conductive (although still quite conductive at 70%). Lead is extremely poorly conductive at 7% and would be quite useless as a test metal -realistically aluminium or failing that copper would be the closest to gold to use in tests. The other metals do not occur naturally in the ground (iron metal is not present in the ground, it is actually iron oxide that occurs in mineralised ground). I cannot find an IACS table for minerals, but I do have some resistivity values (the inverse of conductivity but measured in different units to IACS values). This can be compared with the same values as those for pure metals, thus giving some correlation with IACS values.
Graphite, which is a significant component of black slate, is 5x10-3 Ohm-m. This suggests to me that hematite (which together with a third iron mineral maghemite is a significant component of "ironstone") has a sufficiently low resistivity (high IACS) to be a problem given that it can be present in a very large volume (so the resultant secondary field will be large because of a combination of its relatively high IACS and its mass). Presumably this gives "noise". The IACS value for graphite is so low (the resistivity so high) that it would usually not be a significant problem even when dealing with a thick bed of black (graphitic) slate - however graphitic slate can be high in pyrite which would increase its IACS so it could give a detectable response in some cases.
Maybe I am talking through my hat, as I am trying to reason this from first principles from uni physics, However if correct, lead would be pretty useless for testing depth responses but copper would be a fairly good substitute for gold since it gives 70% of the respoinse of gold (compared with 7% of the response of lead compared with gold). And wrapping things in foil etc would be totally useless since it would primarily depend on the mass and the conductivity of the metal in the foil (magnitude of the induced field would be small despite its large area) - while the IACS of aluminium is very close to gold the mass of the foil would be negligible. Lumps of aluminium metal would probably be ideal test materials.
