Here's an interesting reply from Harry Francis at the limeonline site to the
question on limestone staining which I had forwarded:

>Yes, Limestones often contain iron, and some may stain. Generally these
>stones are not used on exterior placements.
>
>I would be more concerned with the cement used in the pointing. Many cements
>are made using iron ore slag - the slag recovered from the production of
>molten iron/steel. Limestone/lime is used to remove impurities from molten
>iron/steel, resulting in a thick molten float on top of the liquid material.
>The slag is removed after the steel is poured out of the crucibal. After
>cooling, the slag forms a very hard calcium/magnesium silicate/aluminate
>ferrite (ate?) rock -just like cement clinker. It is interground with cement
>clinker to make "slag" portland cements.
>I think these are the cements that would be most likely to stain.
>
>These cements would likely also contain a relatively high amount of sodium/
>potassium salts --which form soluable salts - and staining.
>
>Solution - make sure the cement you use do not contain  slags interground.
>How to find out ? I thionk talking with the cement companies ---technical
>people - as to who is using these by-products in the cements they produce.
>
>Contact with CTL labs in the Chicago area might give some info. Bob Nelson
>of Robert Nelson & Associates (847-882-1146)may give some insights. Nelson
> is a research and testing lab in the chicago area. I will contact him
>tomorrow, if he is available.  Also: Tim Conway (313-529-2411) Research
>Director of Holnan Cement; and James Nicholos, Masonry Market/Technical
>Manager- Blue Circle Cement (404-792-2361). I think all will be happy to
>discuss the issue.
>
>Harry.

John Mascaro