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Darkening of La Grosse Blanche


Well-Known Member
I bought a La Grosse Blanche last year from a well respected vendor. I read on all about the way the turn color when exposed to sunlight, or air, or both. Mine never did.
We endured a long, and very hot summer, so this particular stone spent a few days on the deck in full sunlight. Still no darkening I could see, so it got put away because I became enamored by the La Petit Blanche I bought from my friend Gary. I never had any doubts that the stone was what it was sold as, and really didn't place that much importance on it, but I just wondered why...
I decided to buy boxes for all the stones that did not come with one, and the boxes arrived today. I found the La Grosse Blanche to place it it's new home, and I was astounded to see a darkened stone!
Now, I recall from my readings here that it's uncertain just why this layer darkens when exposed over time, but to me, the darkening was reminiscent of good old rust. I also claim no expertise in the primordial soup that eventually formed our coticules, and I have no degrees in chemistry or geology, but i came to wonder if perhaps minute amounts of iron could be mixed in this particular vein that could possibly account for the darkening.

Any thoughts?



Well-Known Member
I know traces of iron are present within Coticule rock, and much more in BBWs. But if I may take a guess, I would hunch that iron is always found in oxidized form in an age old natural stone.
But whatever makes a La Grosse Blanche turn dark, it has to be exposure to something the rock wasn't exposed to when is was still surrounded by other rock. The most likely candidates for that exposure are air and light.

I'm sorry that I can't offer anything more than what you already knew.

Kind regards,


Well-Known Member
My La Grosse Blanche was darker before I lapped it. It has not returned to the its former darker hue.


Well-Known Member
I have a LGB that I purchased lapped and for the life of me have not been able to turn it dark at all. I have left it outside for a few air..nothing has changed its color.


Active Member
Mine also went from golden brown to yellow when I lapped it. I did find a little bit of red on it that had been hidden under the "tan". Whatever color it is, its still my best finishing stone.


Well-Known Member
Sorry, I know its an old thread but I was reviewing this again. If air and light aren't darkening the coticules then can't we just ask Maurice what causes this layer to darken? I would think he'd know more than anyone else...