This game is a sequel to Jewels of the Oracle, except that is plays more like an alternate version of the first game. It does not follow chronologically, and can be played without any reference to the first game.
Like the first Jewels, this is a first-person game where the player is investigating an ancient civilization. Unlike the first game, this game centers around the Gems of Darkness, not the Jewels of the Oracle.
And, much like the original Jewels, this game is all about the puzzles. There are new puzzles, but players will remember old favorites from the first game. In fact, some puzzles are so similar to the puzzles you had to solve in the first Jewels of the Oracle, you will think you are playing the first game again.
If you are looking to find a copy of the game, it has a surprising number of names it can be found under:
Jewels II: The Ultimate Challenge
Jewels of the Oracle II
Gems of Darkness