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Jewelry, Gems, & Minerals/Does the Flagstone I purchased contain Iron/Rust?


I purchased about 70 lbs of Flagstone from a Nursery yesterday to use in my 55 Gal. Aquarium.  I noticed that some of the rocks had a reddish/brown color, but doesnt rub off in water or stain the water or fingers when touching it.  Could this just be clay or the color of the rock?  Any specific way to test if it is Iron/Rust?  And will it harm the fish?  I suppose I could put the rocks in a bucket with cheap fish and see how they do, but would like to test  the rock any other way.   I read somewhere that Flagstone may contain iron/rust, but that it is Inert?  Meaning it wouldn't leach into the water?  Any advice you can offer on how to test it would be greatly appreciated!  I've attached a pic, if that helps.

I do not know much about fish, but here are some precautions you may want to take. Rust alone is rather inert and should be no problem. However, if there is iron pyrite in the rock that has caused the rust to form, you would liberate sulfuric acid in water which would lower your pH and might harm the fish. Also, if the rock is carbonate (Limestone, Dolomite) it will dissolve very slowly in water, especially if carbon dioxide is present. This would change your water hardness over time. Depending which type of fish you are trying to keep, that could be a problem.
Here are some suggestions: First, soak your rocks in a bucket of water and scrub down well with a brush, then rinse. This will remove lose particulates and most surface coatings.
Take a small rock sample (rock chip) and place in white vinegar. If it start fizzing, your rock is carbonate and is probably not suitable for a freshwater aquarium with sensitive fish but might work well in a seawater aquarium. After all coral are composed of limestone.
If you are still worried you could also preleach your rock. Put them in a bucket with water and let them sit for a month. Discard the water and rinse. This should remove many solubles and make the rocks better suited for your applications.  

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Uwe Richard Kackstaetter, Ph.D. (Dr.K)


I can answer questions concerning minerals, mineralogy, gems, metals, and anything that has to do with geology. However, I am NOT a jeweler. Questions about values, settings, gem stone cuts and appraisals are best directed to other experts on this site. I can however aide in the identification of unknown mineral materials. As a public service and part as training for new geoscientists, our university department provides FREE mineral identification for individuals. Please contact me for details or go to for details..


I am a professor of applied geology and mineralogy with many hours of field experience. Furthermore, I enjoy recreational gold prospecting and mineral collecting. As a professor I am engaged in research concerning minerals and their occurrence.

Member of the GPAA (Gold Prospectors Association of America) as well as the Association of Environmental Geochemists. Member of the GSA (Geologic Society of America) Member of the AIPG (American Institute of Professional Geologists)

Here is a small sampling: Mineral-rock handbook: Rapid-easy mineral-rock determination : written for anyone interested in minerals and rocks - Proctor, Peterson, and Kackstaetter;Macmillan Pub. Co. (New York and Toronto and New York) Physical Geology Laboratory e-Manual [CD-ROM], Kackstaetter, Earth Science Education LLC Colorado Front Range Self-guided Geology Field Trips, Kackstaetter,

Ph.D. in Applied Geology and Mineralogy. I am actively teaching courses in mineralogy and a variety of field courses with mineral collecting opportunities. Background in precious metal exploration.

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