Hi Michael,
Pure diamond can be transformed into carbon dioxide gas at temperatures above 1,652F (900C) when sufficient oxygen is present. Graphite will burn above 400C
Using the most common MAP Gas Torch fuel (for example: Bernz-o-matic), you can achieve “in-air” flame temperatures above 3,700F
You might have vaporized any exposed diamond and/or graphite material in those nodules, if sufficient heat and free oxygen was present.
It is really hard to know what that material is or why it behaves the way you observed while thermal stressing them.
Is the 3.3 density a weight measurement or is it the specific gravity?
Where were these found? I seem to recall a conversation of similar material (images) that someone found near a parking lot before it was paved. I don’t remember if that was you or someone else’s topic.
Cheers!