Seachem Matrix: Pure or Cut?

[Kungpaoshizi]

(Just an FYI if you're thinking about trying this, they label it at ~700 m^2 surface area per liter I think?)
Anyone ever used this stuff?

The other week I ordered some SeaChem Matrix to explore new things...
Then I was also looking into Pumice for the internal surface area just as some extra rocks in the tank..

But then I received my Matrix today..
Read in one forum today that a kid added some to his tank and all the fish were dead the next day..

Then I received it, and it definately looks like just plain old white pumice, but I found this interesting, and perhaps it explains why the other guy had a bad experience..




This came from 1/4 of the 2L jar, so maybe if the whole thing was dumped in and not prewashed...
Anyone familiar with geology off hand that can identify what those rocks are?
They feel kinda light, so I wonder if it's just different categories of Pumice, but like I've seen elsewhere some avoid lava rock because of iron content, so I can only imagine what other hard minerals are mixed in with lava/pumice..

Most sunk, some floated, but the majority was the size of a marble, and looked pretty good.
Either way I removed this stuff and should have a review on the pure white~