Without seeing pictures it is difficult to be sure, but it sounds like a common issue with some slate. Basically it is a naturally occuring spot, a ball of impurities in the stone that when cut into slices (tiles/slabs) creates a circular like splodge. It is basically a concetration of similar minerals that are not the same colour as the rest of the stone, often a bit softer so can be more dull in the slab (as they don’t polish as easily, the harder the material, the more glass like, the softer the material, the more like dried mud etc).

Slate is a metamorphic stone, meaning it used to be something else (a mixture of different minerals and often a lot of clay/silt/mud etc), then it was subjedcted to long periods of pressure and or heat and 99.99% was phyically changed but there may have been pockets of other materials or impurities the stone that were like little balls or clusters, and maybe they reacted to the heat and pressure at a different rate – a bit like having a few chocolate chips in cookie dough or cake mix – they are still in tact as choc chips in the baked cake.

EDIT: “I just spoke with a geologist friend and he says that the spots are not so much, pockets that did not metamorphose as I speculated above, but more that certain minerals within the mix for whatever the slate was made up of, during metamophosis, seem to be drawn togehter and coalesce to form these spots, they do not know exactly how or why, but the result is the same – natural spots”

If this is what they grey spots are, then I am afraid they are a naturally occuring (and therefore normal) part of the stone. If this is the case then you will not be able to remove or even disguise them. Your only argument may then be that a) the sales people did not adequatley warn or inform you about the possibility of recieving a number of these as being ‘normal and reasonable, or b) the samples you saw were all ‘perfect’ possibly selected to show those that are blemish free, and therefore one might argue, not representitive of the actual tiles you were about to buy.

If you wanted to challenge them on this, I would first check their displays and any quotes/paperwork or contracts you may have singed, if their displays do show some of these marks, and/or if you have signed some document or they can point to signs or posters in the showroom etc that point out somehting like :”all natural stones are naturally variable and some may have marks or blemishes etc…..” – or words to that effect, then youu may have a hard time persuing it.

Sorry I cannot be more helpful

Ian