Usually about 1 cup per gallon is enough.  Soaking for two days is also usually enough.  Are you sure the "grey" is a stain or possibly the color of the rock?

It depends on what you are cleaning.  Some specimens will be ruined in Iron-out or any other chemical,  Most carbonates can be dulled by acids, or destroyed.  It will not hurt quartz.  Most specimens are best cleaned with water and a brush.  I talked to one person who Iron-outed everything!  He wasted a lot of money and ruined many specimens.I only use about half a cup per gallon and for me that is a very strong solution, most times a quarter cup is plenty.   Use less and let it soak longer.  Hot water works much better, taking less time and acid. If the water turns yellow, pour it out and rinse the specimens.  No need to waste expensive Iron-out, it doesn't take much.  What were you cleaning?  Did you thouroughly scrub them in advance?  Make sure you know what the acid will do to your specimen before you use it, unless it's just junk (then why clean it at all?).

Iron out is a mild acid.  It will probably not damage the calcite if you use a weaker solution.  What minerals did you iron-out that turned grey?

I can't fathom how a 3 hour soak in iron out could turn white quartz grey.  A total mystery to me.  Seems like there must have been a lot of clay or other material on the quartz, otherwise it doesn't seem possible.

Post a picture of the "grey" rocks so we can try and figure out whats going on if you can.

When I first used Iron Out, I ruined some rocks. My mixture was too strong, and the water too hot when I put the rocks in. It kind of crazed them like old pottery, and turned the quartz grey. I think I "burnt" them, sort of.  Now, I use a very weak solution and repeat until they are clean.

Rebecca wrote on Nov 24^th, 2012 at 5:02pm:

My luck that it what I did to it...I am wanting some for vases and some to redo the bottom of my small pond.