Have used this stone on a garden wall. We went with a grey stone with
a brown/ rust running through it. After a year or so we noticed there was a lot more rustie colour than when first put up, as time went by the rust started to run down the wall in streaks. When i enquired what was happening to my lovley wall i was told it was iron oxide which came naturally in the stone and there was'nt much we could do about it. We are currently building and am not goind to get caught out this time. We are using a stone cladding from fernhill stone looks great like my previous stone wall but the rust will not run we have been guarenteed and have been to see some of the work they have done in the past, very pleased. Hope this might be of some help.. their site is
www.fernhillstone.com if u want to see for your self