I have been seeing a lot of this lately.
I am on my way to go poultice a large area of rust now-
I didn't want to even get involved as it is the same situation that Justin has with a contractor.
These things very rarely have a good outcome-If stone is improperly specified the simple truth is rip it out or learn to live with it-

On Fri, Oct 2, 2015 at 7:42 AM, Fred Hueston <fhueston@gmail.com> wrote:
There is one way to tell for sure if it is iron.  Pick up some Alpha RSR 2000 and place a small dot on the yellow spots. If it turns purple in a few minutes than it is iron. This is one of the testing techniques I teach in my stone inspector and troubleshooting seminar.


On Fri, Oct 2, 2015 at 12:39 AM, Randy Frye <rfrye@comcast.net> wrote:
iron oxide in the stone is oxidizing wicking to the surface. very common with that stone.


On Oct 1, 2015, at 9:04 PM, Justin <justin@jmcstoneandtilecare.com> wrote:

Hi partners
A contractor has contacted me about marble yellowing that he has installed about 6 months ago. When I hear that I think rust iron stains but he thinks there yellow not rust stains. These are the photos he has sent me they're not the best but I can see in some of them some of the discoloration .  Any feedback on the symptoms I told him it looks like iron rust stains, and would a poultice be the best aim of attack. He has his theory's, but I want to get your guys opinions. 
Thank you

Justin Cameron
JMC Stone and Tile Care

Begin forwarded message:

From: Mark Parsons <markparsons@mcparsons.com>
Date: October 1, 2015 at 5:32:44 PM PDT
To: "justin@jmcstoneandtilecare.com" <justin@jmcstoneandtilecare.com>
Cc: Mark Parsons <markparsons@mcparsons.com>
Subject: FW:

Justin, attached are some photos of two areas in the house, island counter to in kitchen & tub deck in master bath.  This was installed around 6 months ago, there are several other counter tops in the kitchen where this is not occurring.  Take a look and let me know what you think.  The area in the master on the tub deck appears to be getting larger, more area turning yellow.  It was only cleaned with water & de natured alcohol, very little alcohol.
 
<WP_20150930_001.jpg>
<WP_20150930_002.jpg>
<WP_20150930_003.jpg>
<WP_20150930_004.jpg>
 
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Frederick M. Hueston
www.stoneforensics.com
office             321 514 6845      
January 18-21, Las Vegas Stone Inspection and Troubleshooting workshop
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Regards,
Stu Rosen
201-446-1200

"EVERYTHING MATTERS"