Raw adamantine is the same as adamantine metal.
To be more explicit, in the case of confusion:
A stone statue made out of an ore will have the same value as a metal statue made out of... a metal. Metal furniture takes three bars, but produces no more value than the stone statues made of ore do*. Even more important, you will probably have a mason with higher skill than you will have a metalsmith early in the game. Especially with something ultra-valuable like adamantine, you're better off getting your masons to just carve two or three statues.
* edit: HAHA! I was lying! I forgot that Adamantine is special in this regard, as well. Adamantine ore is only 250, instead of the metal's 300. It's still rediculously valuable either way, and still better used as an ore, though.
Statues have a base value of 25, times the quality modifier, times the value of adamantine, which is
300 250. To make a royal bedroom, you need 10,000 DBs in value. An adamantine ore statue has a minimum value of
7500 6,250. That gets multiplied by quality, so if your masons are anything above proficient, you have "Royal" in the bag. You do need to have either several rooms elevated to royal level, or you need to make a combined-function room with several adamantine statues to boost the value to compensate for the value drop of a single shared room.
Is it guaranteed to get the king with adamantine? My last fort found adamantine and I never noticed a king, though it perished a few seasons later thanks to the nemesis bug.
Although it may be that I had already reached max pop without a baron.
If you got a nemesis bug, it may just be that the file that had the data for the king was deleted. (In fact, his arrival may have been what sparked the nemesis bug.)