Frederick M. Hueston, PhD  

Stone Care Consultant 

It was one of those cold Florida mornings; it must have been 62 degrees. Of course, my friends up north would probably call it a heat wave.         

I decided to put on my leather jacket and hopped on the Harley for a bracing ride to my favorite greasy spoon. My plan for the morning was to get a cup of Jo and listen to all the regulars complain about the weather. 

Of course, just as I was about to head out the door the phone rang.  Thinking so much for my plans, I answered it on the third ring. 

The voice on the other end was really nervous, high-pitched and squeaky. To top it off, the caller was a guy.  

He explained that he had just installed a granite countertop and that his customer was complaining about it etching with some spilled vinegar. That did not surprise me, since there are some granites that contain calcium binders. 

Let me side-step here and give those who have absolutely no idea what I am talking about a little stroll down “Geology Lane.” 

Marble and certain limestones are composed of calcium carbonate as its main mineral. Acids will react with the calcium carbonate and cause the stone to etch (dull). Granite typically is composed of minerals, Quartz and Feldspar crystals, which will not react to most acids except Hydrofluoric acid (HF). 

So when a mild acid such as vinegar etches granite, you have to scratch your head and wonder why. The only explanation is that the granite contains calcium. I am seeing more and more of this problem with certain granites, especially some of the black absolute types.  

What the caller told me next had me baffled. He said he took a piece of the granite and placed some Muriatic acid on it and let it sit for a few minutes and it did not etch. When he placed the vinegar on the same piece, it did etch. 

I asked him to tell me this again ’cause I couldn’t believe what I was hearing. He repeated it and, lo and behold, my hearing was just fine, however, I was just as confused as when I was a little kid and saw my mother kissing Santa Claus. 

“How could this be,” I thought. So I asked him if he could send me a sample of the granite so I could test it myself. Then I hopped on the bike and headed out for my delayed breakfast.  

My ride was chilly but I kept warm thinking about what could be the real cause of the etching – and was this guy really testing the stone properly?

Three days passed and finally the stone sample arrived. I stopped what I was doing and immediately opened the package to start my testing. I felt like a little kid at Christmastime!

The stone was called Brown Antique and was dark brown to almost purple in color. I placed the stone on my kitchen table and starting foraging in my test supplies for some vinegar and Muriatic acid. I placed a few drops of vinegar on the granite and waited about one minute. I wiped off the excess vinegar and picked up the stone to examine it under better lighting. 

Lo and behold, it had etched the stone. I noticed that the etch was consistent and not spotty. Next, I took some Muriatic acid, placed it on the granite and let it sit for one minute. I wiped it off and to my surprise,  it did not etch. Let me shout this out… IT DID NOT ETCH!  

“Holy cow dung, Batman” I thought, “this can’t be.”   So the next thing I did was...

OK, here’s your cliff-hanger for the month! If you think you know what is happening with this stone, send me an email with your explanation. I will reveal the answer next month.

The Stone Detective is a fictional character created by Fred Hueston, written to be entertaining and educational. He has written over 33 books on stone and tile installations, fabrication and restoration and also serves as an expert for many legal cases across the world. You can send any email comments to him at .