The texture of obsidian, shown below, is best described as glassy.

Why does obsidian contain few or no mineral grains?

Because it cools so quickly that few or no mineral grains formed.

Obsidian contains few or no mineral grains because the material that would normally form mineral crystals does not have time to form a crystalline structure because of very rapid cooling after volcanically erupting into the air or onto the surface. Obsidian is a volcanic glass.