I have noticed that many of the indian sandstone slabs I had laid 3
years ago are developing lots of unsightly dark 'spots', as shown in
these images:

http://www.davidwgreen.talktalk.net/DSCF0173.JPG
http://www.davidwgreen.talktalk.net/DSCF0174.JPG
http://www.davidwgreen.talktalk.net/DSCF0175.JPG

These marks cannot be removed by a pressure washer, and appear to reach
down into the stone itself. I started to notice these stains last year,
but this year there are a lot more of them. Not all the slabs are
affected as you can see in the photos. Can anyone tell me what is
causing this problem and what I can do about it? I'm worried that this
will continue getting worse until my entire patio is black! Is there a
name for this problem?

Is this a case of defective stone? Should I be demanding replacement
slabs from the supplier or the company which laid (and supplied) the slabs?

I hope someone can help.

Sincerely,

D Green
Cambridge UK