Yes Soltan it is Diatomite
Mined in Queensland and their website shows some pictures on the products
http://www.maidenwelldiatomiteaustralia.com/
I will do a test as you suggest on the weekend.
I have a small pot with some in it on the deck and I watered it this morning before work 7am, and it was still damp this evening at 6 pm when I got home so it looks pretty good moisture holding properties.
Ken