Obsidian dragons

Obsidian dragons are the most intelligent of the gem dragons, but also the most dangerous. They prefer to lair near volcanoes, and are notable for their psionic abilities.
Gem dragons are all inherently psionic creatures. They develop telepathic abilities shortly after hatching—sometimes even before—that grow in strength as they age. Some hone these mental powers further, manifesting

powerful magical abilities through their minds.
These psychically adept dragons often find themselves at odds with aberrations, particularly mind flayers and aboleths. The gem dragons are skilled at locating areas where the Far Realm intrudes upon the Material Plane and set themselves against the alien creatures spawned or twisted in such places.

Physical description

They have smooth, black, glass-like scales that had razor edges and can breathe a cone of fire.

Combat

The obsidian dragons were avoided and opposed by the other gem dragon species since that time. Obsidian Dragons are often vicious, selfish beings, not caring even for the lives of their flightmates and children, often killing other Obsidian Dragons to rise in prominence among the flight.
When an Obsidian Dragon encounters sentient creatures, it will often make a split-second decision on whether to address them or destroy them on the spot, usually with its breath weapon.

Lair

Gem dragons set themselves apart from the affairs of other beings, even their own kind. They search out lairs far from established populations which they can tend in solitude, or so they can carefully cultivate a circle of servants or vassals to tend their interests in the surrounding region.
Gem dragons break their natural inclination to solitude to mate and protect their clutches of eggs, but rarely for any other reason.