As mentioned on page 1 I use black iron sand, collected from south of Raglan.
This stuff is very heavy, a 3/4 full 10l bucket will weigh in at some 20kg.
I haven't had any issues with it other than the second lot I got had lots of very fine 'dust' that clouded the thank for about a week. I don't test pH so cant say one way or another how it effects this, and I dont grow plants so cant say how good it is for this - but I can't see that there would be any problems. About it getting into the filter, I have had some get into my cf1200 but it just sits in the bottom - doesn't seem to work it's way up to the impeller.
See http://www.fnzas.org.nz/fishroom/viewtopic.php?f=41&t=50025 page 2 & 3 for photos
ps Depending where it is collected from the colour will range from very dark blue/black to very dark grey to grey. One interesting thing that happens is the iron sand and silica/rock/etc will separate out slightly to give bands and patterning where there is different amounts of current or if you poor it through the water column.