Adamas is an interesting manga. It's tagged as Seinen and it does read like a Seinen manga, but it's got a female main character with a romance subplot as well, so it has shades of josei thrown in.
A big part of Adamas is Reika taking down the big jewelry company that's been controlling the industry for hundreds of years, while picking up side jobs here and there.
The story itself is quite great, with plot points introduced, expanded upon, and resolved in a satisfying manner. Though it's not mindblowing or overly memorable, reading it is always enjoyable.
The special powers are pretty nifty too, with different
characters each having a different power drawn from different gems - crystals help with healing, rubies grant superhuman strength, emeralds enhances the user's perception, and so on. It's neat how the characters try to overcome their opponents using wits, teamwork, and their own power.
The cast is also quite likeable and unique. The main cast is sufficiently developed and even the supporting cast usually have background stories or twists on their own.
And the art is really good. The characters look great with clean lines and the actions scenes are typically easy to follow. Although some characters can look weird at times, especially those of Caucasian descent.
Overall, it's recommended for those looking for a good action/supowerpower manga with a more realistic bent.