I've done some more internet surfing regarding the thermal properties of charcoal and come up with many more questions than answers. It turns out that much of the positive reports come out of China and were written in order to sell charcoal infused fabrics. The charcoal was derived from bamboo.
Quite a bit of cross checking has convinced me that the radiant barrier thing is quite verifiable. But outside of advertising materials, I can find nothing on the conductive nature of granular charcoal alone.
Various tables show that various forms of carbon are all over the map thermally and electrically. Diamond is an electrical insulator, while graphite is a good conductor. Diamonds are the most thermally conductive substance and therefore the worst possible insulating material. So that ends my plan to put 8 inches of diamonds into the attic.
Other carbon materials have different rates of transfer at different temperatures.
Carbon has more forms and it forms more compounds than any other element, so the quantity of information and misinformation is daunting. The internet is full of scientific and purely speculative “information” on the subject.
When a Google search mentions charcoal and insulation together, this thread pops up on the first page right alongside information from peer reviewed scientific journals based on laboratory testing. When I type in “granular charcoal insulation” The first three listings are this thread and others on this site where I have linked to here. During this last surfing session, my own input has popped up about 20 times. I've got to start marketing some bogus product. With the top three search positions, I could make a fortune !!!
So, since according to a Google search, I am one of the world's leading experts in the field, it is incumbent upon me to conduct an experiment. --------------------------------------------------------------------------------------------
THE EXPERIMENT – I don't have anything resembling a proper laboratory but I can get my hands on some charcoal, a shoe box and some firebricks. I will grind the charcoal finely and then place a bed of it into the box, followed by a firebrick which has been heated in the oven. The brick will be covered with the powdered charcoal so that top and bottom have an equal covering. Other identical bricks will be heated on the same shelf of the same oven and placed into identical boxes and covered with various insulating materials such as cellulose fibre insulation, sawdust and fibreglass. The temperature will be monitored at various intervals.
This sort of test won't give an accurate R value, but it will make a good comparison between the various materials tested. If the charcoal meets or exceeds the insulating capacity of blown cellulose, then the whole idea is brilliant and I'm a genius. I'll hire myself out to NASA. Should charcoal insulation perform more poorly than sawdust, I guess I'll blame it on flawed testing. I've seen TV before so I know how that works. 
This is going to take a little while to get the supplies together. I'll post photos and numerical results.
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