cough cough, here comes the chemistry student to save the day and give you a lesson on what adamantine actually is.
first, lets not confuse it with the similarly sounding adamantane
adamantane, or properly called tricyclo[3.3.1.13,7]decane is the simplext diamondoid, with the sum of C10H16. the adamantane itself is not particularly useful for anything. however it's easy to recognize structure helps chemistry profs all over the world. "See that structure of the phosphorous? just like adamantine"
also do not confuse it with Amantadine
amantadine or properly called 1-amino-adamantane, is more or less the same as adamantane, except that it has a NH2 group on top of it. that stuff was used against the asian influenza btw.
now for the one that you really want to know about, adamantine
adamantine is an actually existing mineral, usualy refered to as adamantine spar. it's just a brown-ish derivate of corondum. corondum, as some might know, is just your usual aluminium oxide, Al2O3, the very same that forms a thin film over aluminium and makes you think that the stuff never rusts
for the kind of geology that DF shows off, adamantine should only occur in small clusters in gneiss, mica and schist. also, it is a common component in beach sand.
I hoped you enjoyed the lesson.
adamantine is neither very hard nor blue nor in strands or anything.
if you want the fictional metal, in most fiction it is either refered to as adamant, or as adamantium.
why toady chose to name his hidden fun stuff after an actual occuring mineral, well, probably didn't know it existed