Adamas is one of the guitar manufacturers owned by the Kaman corporation. Adamas is the Ovation premium line.
Like Ovation, Adamas produces guitars using the proprietary lyrachord bodies in a unique semi-parabolic shape. Like any other premium maker, a lot of the price difference is in cosmetic detail work and increased quality control standards. Ovation, however, is not a budget brand. They have Applause for that. Ovation makes some fine instruments, especially in their $1,000+ lines. Adamas is not the "real thing" in the same way as Gibson v. Epiphone. It really is a premium product.
Physically, Adamas guitars look just like Ovation guitars. They use the same headstock, the same soundhole designs, and the some rosettes.
Materials are what really set Adamas apart from Ovation and other manufacturers. All Adamas guitars use carbon-fiber or graphite in the construction of their tops, rather than the commonly used sitka spruce and cedar of most other guitars. Most of the models use unidirectional carbon-fiber. Others use a carbon-fiber crossweave, and at least one (the Q) uses woven graphite with a poplar core.
Like all Ovation products, every Adamas guitar is equipped with piezoelectric pickup elements, and some sort of Ovation preamp.
People who use Adamas guitars:
Melissa Etheridge: Both of her signature models -- the white six-string and her twelve-string.
Kaki King: Uses a custom made Adamas. It's blue with a "cloud design" around the upper soundholes rather than the signature Ovation/Adamas feather design.
Preston Reed: (No, not the guy from Equilibrium.) As far as I know he just uses some kind of stock Adamas. It looks like W597, but could just as well be an older model.