Well, if you search a little further down the postings here, you'll find a post about using river rocks Vs Iron Shot. By river rocks we generally refer to medium sized, 1 or 2 inch, ovoid and smooth rocks. This type of material is the advanced level for some styles of IP. It is actually as challenging as the Shot, but more because of the shifting surface which presents an occasional edged surface. (These hurt when they land between knuckles, IMHO more than steel shot.) The argument is that a student is conditioned for striking surfaces with variable angles presenting, like the bony spots on a foe's body.
The drawback with stone vs shot would be that the shot develops a denser, harder hand with more peripheral conditioning, like I said, the wrist, elbows, shoulders, etc. So, theoretically, one is more applicable to developing a fighting hand, the other to developing a breaking hand. I don't really think one way is better than the other for developing IP, just that the strengths of each method are different. If you want to learn to hit with Iron Palm power, you have to take the time to study the strike itself. Otherwise, you're just making your hands harder, and you really don't need full on IP conditioning to do that. You could take up masonry or carpentry as a hobby, for instance. You'll have really hard, tough hands, but it won't do much for your fighting ability or striking power. This isn't meant to sound sarcastic, I'm just saying that IP without studying the strike and power generation is going to result in an empty skill, and you can get the effect without the skill.
My logic in the tumbled hematite is to combine the Iron with the rocks. This would be the shifting uneven surface melded with the Iron. Hematite is an iron oxide ore, known as iron sand in some places. What I propose is that using the larger pieces of smoothed out hematite (Which sheds an oxide dust,Ie rust) would combine the two methods. I'm thinking ovoid pieces would be best at about one inch diameter or less, but still larger than regular gravel. Just to be certain everyone here realizes this- I Never learned this from anyone, it's a personal experiment born from the evolution of almost thirty years of iron arts study. I also think that tumbled hematite is easier to find than Iron Shot, which is as rare as frog's hair these days.
I figure it may take a while to see what the training does, but when I have some definitive results, good or bad, I'll post them here. I only just got the stuff together for trying it out, so I have no idea what if any effect this medium will have on my hands.
Whatever works- works.