A rock with no grains or a glass-like texture is formed in what environment?

The correct answer is - water environment.

When a rock doesn't have any grains, or has a glass-like texture, than it means that that rock has been formed with very extreme and quick cooling. This kind of cooling is possible only in the water environment. Once the lava gets in touch with the water, it starts to cool off very quickly, and in a very short span of time an igneous rock is formed, but unlike most of the igneous rocks, this rocks will lack grains, or will have the glassy appearance because they didn't had enough time to form any grains.