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Cleaning Minerals and Fossils
A general overview

By: Christian C. Burke

It is a great feeling when you find some cool new specimens to add to your collection.  Now that you've got them home, you want to make them look their best.  There many methods for cleaning and preparation, and lots of tools available.  Let's examine some of these:



Perhaps the most popular cleaning tool known to man is the good old fashioned brush.  Often times, a little water and a scrub brush is all that's needed to clean your specimens.  There are endless styles of brushes on the market, and some are more useful than others.  Old toothbrushes are a favorite with collectors, as are plastic-bristled household scrubbers and dish brushes.  For more vigorous cleaning of harder materials, wire brushes may be useful.  For delicate cleaning of fragile pieces, small paint brushes may be better.  Generally speaking, however, brushes are useful only to remove surface layer dirt.  To remove tougher material, such as mineral crust and matrix rock, you will need to explore other options.


Another very popular cleaning tool is the metal pick.  Picks are useful for scraping crust and dirt out of crevices, and for removing small amounts of matrix rock from around crystals or fossils. Inexpensive pick and probe sets can be purchased at any hardware or automotive store.  Used dental picks can often be obtained from electronic or mechanical surplus stores.


For removing large chunks of material and trimming specimens, a set of cold chisels and a hammer are often a good choice. Remember to use protective eyewear!

Other Hand Tools

Various other implements may come in handy when cleaning specimens.  Pliers can be used to crush or snap off protruding pieces of rock.  A vice may help hold a piece steady while using some other tool.   There are so many types of tools available on the market that their uses are limited only by your imagination.


Dremel (Rotary) Tools

A good Dremel set is a nice thing to have around any household, and the collector will find their own uses.  Diamond grinding bits can be used to carve matrix material away from the specimen to improve presentation or expose an embedded crystal.

Cleaning Guns

High pressure spot cleaning guns are electric tools that shoot a powerful, pinpoint stream of water or cleaning solution.  Primarily used in the textile industry for stain removal and cleaning silk screens, they are also outstanding for cleaning mineral specimens.  These are serious tools, powerful enough to puncture skin, and are great for removing crusty mineral deposits, forcing deep iron stains out of cracks, and even carving and chipping away softer matrix materials

Bead Blasters

The micro-abrader or "bead blaster" is the ultimate cleaning machine, hands down.  It is an air-driven tool that propels a concentrated stream of of tiny abrasive particles at high velocities to quickly erode away unwanted material from mineral specimens, fossils, etc.  They are used extensively in the dental industry, and also by museums and mineral suppliers.  Different types of abrasive powders are used for different applications (ideally the abrasive media should be harder than the matrix material but softer than the specimen itself).  For cleaning minerals, the most widely used media consists of tiny glass spheres.  For softer minerals, powdered dolomite, sodium bicarbonate, and crushed walnut shells are also used.



For regular old dirt, sometimes a little soap is all you need.  Calgon...take me away!!!


Some minerals are water-soluble (e.g. halite), and therefore cannot be cleaned with water.  In this case, a swab dipped in rubbing alcohol may be used for cleaning.


A saturated brine solution may be used on certain water-soluble salt bed minerals (halite, hanksite, borax) to avoid dissolving a specimen or damaging the crystal faces.  Many people who collect such specimens (e.g. from Searles Lake in California) obtain the original brine water from the site to clean their specimens, which is safer than using water alone.  With most of the water molecules already involved with the dissolved salts, they are not as available to attack the mineral specimen.

Oxalic Acid (C2H2O4 · 2H2O)

An organic acid, derived from plants (the stuff that makes rhubarb leaves poisonous) and also produced in animals as a metabolic by-product of vitamin C.  Sold in powdered form as "Wood Bleach" in hardware stores.  A solution of oxalic acid is very good for removing iron oxide (rust) stains from minerals.  Works best when a little heat is added.  I use a programmable crock pot to cook my specimens overnight, then let them cool slowly to avoid cracking.  

Oxalic acid won't eat the flesh off your skin, but it is poisonous and can be absorbed through the skin.  It's a good idea to use gloves and eye protection when handling this chemical...though it is more dangerous in powdered form (avoid inhaling dust).  Will etch some minerals (e.g. calcite and fluorite) depending on strength and soak time, so always test on a mediocre specimen first.

Hydrochloric Acid (HCl)

A highly caustic, corrosive, and reactive chemical, HCl is used to remove lime from concrete surfaces and to lower pH in swimming pools.  Sold as "Muriatic Acid" in hardware and pool supply stores.  Quickly dissolves certain minerals (especially calcite, with which it will react vigorously) and organic stains.

HCl is probably one of the more dangerous chemicals you can buy without a license.  Extreme caution must be used when handling, storing, and disposing of this chemical.  It will burn your skin, cause permanent eye damage, and the vapors alone can kill you.  But the most dangerous property is that it can react violently with other chemicals, causing a nasty explosion.  Gloves, eye protection, and a respirator are a must...and always have PLENTY of neutralizing agent available (e.g. sodium bicarbonate, calcium carbonate).  In my personal opinion, HCl should only be handled by people with a basic understanding of chemistry.

Hydrofluoric Acid (HF)

One of the most corrosive and toxic acids known to man, HF is used primarily for industrial applications.  Sometimes used to expose minerals (especially fluorite) from surrounding matrix rock.  

HF is so incredibly dangerous that it should never be used by an amateur, and therefore will not be discussed here further.


In many cases, the appearance of a mineral specimen or cut stone can be greatly improved by the use of fixatives, sealants, or coatings.  For example, emeralds tend to be rather porous stones and frequently have air bubbles or small cracks that reduce the their clarity.  Because of this, nearly all cut emeralds sold today are treated with filling agents such as oils or polymers.  Likewise, it is common in the mineral business to treat or coat certain minerals to improve their appearance.  While there is some ethical debate over this practice, it is far more common than most people realize.  In the gemstone trade, it is considered acceptable as long as any treatment is disclosed to the purchaser.  This has not been firmly established in the mineral business - though perhaps it should be.  However, the purpose of this article is not to discuss ethics, but simply to provide information.  So, here are some of the more common treatments and their applications:


Perhaps one of the most common treatments is the application of a silicone based spray.  This is often a food-grade product, used for lubrication and preventing corrosion in food processing machinery.

If you are having trouble with finding silicone lubricant sprays then think about sprays from companies like WD-40."

 Also used is standard silicone lubricant, found in the automotive department of any hardware store.  Either way, the purpose is the same - to maintain a moist appearance.  This helps enhance the color of the mineral and reduce the visibility of etching, scratches, and dings.  Minerals treated with silicone eventually start looking dry as the coating wears off, gets absorbed, or becomes covered with dust.  In this case, it is necessary to clean the specimen and reapply the silicone spray.  If not cleaned, a residue can eventually build up and make the specimen look dull.


Various different natural and synthetic oils are used to treat minerals (cut stones in particular).  Commonly used are cedarwood oil, Canada balsam oil, and mineral oil.  When used as a filler in cut stones (e.g. emeralds), the oil selected will ideally have a refractive index that is close to the stone being treated.  This ensures maximum clarity, and makes it more difficult to tell it has been treated.  Treatment of porous stones is usually done in a vacuum chamber, to remove air bubbles inside the stone and allow the oil to fill in.  Oils are also used to treat mineral specimens in the same manner as silicone, but are especially useful for preserving minerals that react with air or cannot tolerate being dehydrated.  Notable examples include hanksite (will disintegrate in dry air) and borax (colorless and often quite clear when formed, but turns powder white after being dehydrated.


Waxes are used to enhance the luster and color of certain minerals, especially semi-porous ones such as feldspars.  The specimens are dipped into liquid wax, the excess shaken off, and then allowed to dry on blotter paper. When done properly, it is very difficult to tell that a piece has been treated.  One example of a mineral commonly treated with wax is amazonite (like those from Colorado and Ethiopia).


Clear plastic compounds are often used on gemstones (e.g. emeralds) as fillers and sealers.  They are also used to repair cracks, or to provide a high-gloss shell on stones that don't normally polish well.  By far, the most popular of these treatments is a specially formulated epoxy resin called Opticon.