Silver granite coping cement jointing stains - Staining from granite coping joints

[Boy Wonder]

We have been using/laying silver granite for paving and wall coping for years with no problem. On a recent project we noticed dirty, almost rust-like water stains running down wall from joints in coping, much worse than the usual slight discolouration expected. We have replaced jointing in one section as a test but problem still occurs. Anyone else experience with this?

[Tony McC]

Did you ever resolve this? Is there, perhaps, a vein of iron running through the affected piece of granite?