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Hot Rocks

YOUR GUIDE To Modern Metal Detecting

Magnetic Rocks

The Information Cache

Brought to You by Casanova's Metal Detector Center - EST 1979  

"Hot Rocks" Can Be A Major Problem For Some People Using A Metal Detector, And Cool Collectables To Others.

Metal Detectors Respond To The Presences of A Magnetic Field.  Natural Occurring Magnetic Minerals Can Cause Problems For Anyone Using A Metal Detector.  Rocks Can, And Are, Dug Up By Metal Detector Users.  Most Of These Rocks Can Be Avoided With The Proper Use Of A Quality Metal Detector Designed To Avoid These Minerals Properly.

From A Different Point Of View, A High-Performance Metal Detector, Properly Designed For Detecting In Highly Mineralized Environments, Offer Circuitry-Detection-Control To The Point That Offers A Mineral Collector, Rock Hound, Meteorite Collector, Gold Nugget Prospector, Copper Nugget Prospector, Silver Nugget Prospector, etc.., The Perfect Machine To Detect Such Targeted Treasures.


Mineralized Rocks / "Hot Rocks"

Casanova's Reference Material

"Hot Rock" is a coined phrase given to mineralized rocks, by the metal detecting industry, that causes a metal detector to respond.

Mineralized rocks can be detected by a metal detector because they are different in magnetism than the surrounding body of ground contents.

Negative - Hot Rocks

Such as Magnetite, tend to give a "booing" sound when the searchcoil is passed over them.  The greater the difference between the rock and the surrounding ground, the louder the "booing" detection sound from a metal detector.  An Iron Oxide.

Positive + Hot Rocks

Such as Maghemite, tend to sound just like any other metal target.  Positive hot rocks can give the same detection sound as a nugget and can give a "zip-zip" sound. Positive hot rocks will test any prospectors patience.  An Iron Oxide.  Commonly used in magnetic tape, for example in the magnetic layer of audio tape, floppy disk, digital tape, etc.. The particles can be magnetized to represent data.



Magnetic Minerals

Magnetism occurs when there is an imbalance in the structural arrangement of iron ions.  Iron is found in two principle ionic states; called ferrous and ferric ions.  The ferrous ion has a charge of positive-two; the ferric ion has a charge of positive-three.  The two ions have different atomic radii.  The higher-charge of the ferric-ions pulls the electrons surrounding the ion in tighter, making for a smaller radii.  This fact can lead to the different ions being placed in separate positions in a crystal structure.  Electrons that move from the ferrous to the higher positively charged ferric-ions create a slight magnetic field.

The minerals that are magnetic, range in magnetic strength from being capable of lifting small steel objects to barely turning the needle on a compass.

A few minerals may not be magnetic, but are still attracted to magnets.  Once a specimen is established as magnetic, identification becomes a rather routine exercise.

A Quality Compass As A Field Test Tool - Magnetism is somewhat of an unreliable property; as not all specimens may demonstrate magnetism.  While the presence of magnetism may all but clinch an identification, the lack of magnetism should not generally exclude typically magnetic minerals.  A high-quality compass needle is a good test-device for testing magnetism, as is a magnet on a string that might sway near the specimen.

The Common Minerals That Demonstrate Magnetic Properties


  • Magnetite; Strongly

  • Maghemite; Strongly

  • Pyrhotite; Sometimes Strongly; Inconsistent

Attach To Magnets

  • Iron-Nickel, Attracted to Magnets


  • Babingtonite

  • Chromite

  • Columbite

  • Franklinite

  • Ilmenite; weakly; always when heated

  • Manganbabingtonite; very weak

  • Platinum

  • Tantalite

In Regards To A Metal Detector, All Of The Above Become An Issue To The Professional Metal Detector User When Searching In Ground-Buried Searching Applications.


Magnetite; A Natural Magnet

Magnetite is a mineral with a defined element structure.   There Is A Combination Of Two Different Metal Ions Occupying A Specific Location Within The Structure.  The Metal Fe +2, And, The Metal Fe +3, Are Two Different Metal Ions Within Specific Locations Of The Structure.  The Two Different Metals Causes A Transfer Of Electrons Between Them In A Defined Direction (Vector).   This Electric Vector Generates A Magnetic Field.  This Magnetic Field Is What Is Detected By A Metal Detector.

A Common Name For Magnetite Is "Lode Stone".  Magnetite Will Stick To A Magnet.

Another Common Name For Magnetite Is "Black Sand".

Gold Prospecting Note.

Gold Nuggets And "Black Sand" Are Commonly Found Together In Many Parts Of The World.  Gold Nugget Prospectors Can Use A Magnet To Pull Black Sand From Their Gold Pan Concentrates, Leaving The Gold Dust Behind.


Color is black

Luster is metallic to dull

Transparency: Crystals are opaque

Crystal Habits most commonly found massive or granular

Hardness is 5.5 - 6.5

Specific Gravity is 5.1+ (average for metallic minerals)

Streak is black

Chemical Formula Fe++Fe+++2O4

Empirical Formula Fe3+2Fe2+O4

Associated Minerals are talc and chlorite (schists), pyrite and hematite.

Other Characteristics: Magnetism stronger in massive examples than in crystals, striations on crystal faces (not always seen)

Notable Occurrences include South Africa, Germany, Russia and many localities in the USA

Best Field Indicators: magnetism (Magnet Sticks To Magnetite; Compass Response; Crystal Habit and Streak

Environment Common accessory mineral in igneous and metamorphic rocks.  Can be biogenically produced by a wide variety of organisms

by Gene Casanova

One Of The Original 'Metal Detector Industry's' Dealerships

- Offering Detector Since The Beginning, In 1940s! -

Honest Industry-Insider-Information, Equipment Experience And  Application Skills Development!


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