An extremely small percentage of meteorites (far less than one in a thousand) do not show strong attraction to a magnet. They are so rare that we usually discount anything that will not adhere to a magnet. Those meteorites look similar to volcanic rocks from Earth, and are not metallic in appearance.
http://www.aerolite.org/found-a-meteorite.htm
Here's some actual advice from the great Reg Sniff from 14 years ago
Posted by Reg Sniff on 1/2/2002, 7:57 pm , in reply to "Re: Shootin' Star Shootout (long post)"
Hi Jim,
I wouldn't be too concerned about frequency of a detector and how it affects the detection of meteorites simply because each machine has a different design as to the level of sensitivity of the different modes. If all things were equal then it is possible that a lower frequency detector probably would have a slight advantage when detecting iron and stony iron meteorites. However, since detectors vary considerably in many factors including the sensitivity level, the frequency factor is lost.
Most manufacturers develop the all metal and disc sensitivity to suit the primary purpose of the detector. For example, a detector designed for coin hunting will normally give a nice strong response to a coin but won't be so intense that it overloads the audio. This gives the operator some feel for the size of the object.
Nugget hunting machines, by nature have greater sensitivity so they provide a louder signal to small gold objects. As such, they will provide a stronger signal to stony meteorites also.
PI's work well for meteorite hunting. One of the more popular brands are the Minelab SD's. They do not produce quite as strong response as a good Gold Machine to a stony meteorite, but because they do not respond to the ground or to most hotrocks, they can actually be superior. Other PI's such as Eric Foster designs also work quite well, providing the delay is set to minimum. At 15 usec, they will give a decent signal to a stony meteorite and since they also ignore the hotrocks, many of the questionable "hotrocks" are ignored. However, one should realize that increasing the delay will cause the meteorites to be ignored.
To better understand why meteorites give conflicting signals on different detectors one has to understand certain basic theory. For example, if we were to draw a linear graph of all targets a metal detector can detect and try to relate them to something, maybe it would make more sense. If we use a general graph where something like ferrite is at one end and pure silver or better yet something pure resistive at the other extreme and label one end 0 degrees and the other 180 degrees we can sort of relate targets for ground balancing or even discriminating purposes.
Now, if we let ferrite be the 0 degree setpoint, then silver would be somewhere in the 170 degree range. Something like saltwater would be 90 degrees, a nickel would be something like 140 degrees and other coins would fall between the nickel and a quarter.
Ground signals are down in the ferrite range, maybe 0 to 10 degrees or so. Stony meteorites fall either in that range or very close. My guess is that Gold Basin meteorites fall somewhere in the range of 6 to 12 degrees. Iron objects will have a range of something like 20 to 90 degrees depending upon composition and size. Most stony iron and iron meteorites should fall in a range between the ground range and the saltwater range, again depending upon size and composition.
If you adjust a detector to give minimum response to a piece of ferrite then all objects including meteorites should respond with a positive audio response in the all metal mode. If you adjust the detector to typical ground conditions, then ferrite will give you the "negative boing", but most meteorites still should give a weaker positive response. Exceptions would be areas where the ground is strange.
If you have the ground balance set so when you lower the coil, the audio decreases, then some of the stony meteorites will respond with a "negative" signal. Again, this depends upon the actual adjustment of the ground balance control. Actually, one could get a negative response, no response, or even a very slight positive response from a meteorite, depending upon the local ground mineralization. However, I have found very few places where the ground is so strange that proper ground balancing would yield a negative response to a stony meteorite such as those found in Gold Basin.
I am sure there are some stony meteorites that would have less nickel and the iron would be in more of an oxide phase that they could give a negative response on a detector properly ground balanced to the ground, but they probably aren't Chondrites. Chondrite meteorites by nature have iron/nickel nodules which aid in their detection. In many cases they also contain similar composition as the native basalt, which can be classified as one of the leaverites. As any serious nugget hunter knows, basalt is easily detectable.
A couple of quick notes before ending this long winded post. Other leaverites such as magnetite will respond very simular to ferrite. Hematite will be more positive, and other iron oxides generally fall somewhere in between. The stony meteorites are quite similar in response as hematite.
Ground signals vary in both composition and intensity. There are different compositions that cause an audio "ground" response. One scenario might be two areas, one having little "black sand" and another might have a lot of black sand. So, the setting on the GB control that minimizes any audio response from these black sand conditions may be the same, but the actual intensity as seen by the detector may vary considerably. This is one of the reasons, the setting of the GB control as a reference will have little meaning as to the ground strength or even how a detector might respond depth wise. However, the setup I mentioned quite some time earlier for Mark 1 users displays ground intensities and does a much better job.
One final note, I have either hunted seriously with or at least tried some 15 to 20 different detectors on Gold Basin meteorites. All have had fundamentally the same response providing the ground balance is set correctly.
I no longer have my Compass AU 2000 but I am sure from what I remember about this detector, that this machine would work quite well for both nugget and meteorite hunting.
Reg
But it sure sounds fun! If it was a big enough chunk of metal, I'm sure your F70 could detect it down past a couple of feet in SL mode. Something to think about is it might even ring up higher than iron depending on its shape or size, even if made of iron. I'm sure there could be smaller fragments much shallower too or on the surface like Rob said.