The magnetometer is an excellent tool to add to your treasure hunting arsenal. Until recently, mags were very expensive and only the serious treasure hunter would consider buying one. Now, there are several projects which result in very affordable mags with decent performance. I built a fluxgate mag based on Speake & Co's easy-to-use fluxgate sensor. The full report is listed below.
If you are interested in the theory behind the mag I can highly recommend the book The Magnetics Measurement Handbook by J.M. Janicke. Besides a good coverage of magnetics theory, it contains much of what you need to build a useful fluxgate magnetometer, plus other devices. Note - Jack Janicke passed away and his book is no longer being published.
Besides the fluxgate mag, the proton precession mag is also a do-able home project. An early article by Nicholas Wadsworth has been the basis for several projects such as Phil Barnes' PPM. Dan Fountain has also been working on one, and sooner or later I will build one to compare with my fluxgate unit. Visit the Mag forum to see what other people are working on.