You will receive a tumblestone very similar to the ones in the photograph and measure app. 2 to 2.5 cms across.
This is a man-made glass as are most similar items (skulls, tumblestones etc.) on e-bay, a number of which are said to be volcanic but aren't. Although a manufactured glass, Blue Obsidian is still used by healers and is said to be beneficial to the throat, tonsils, thyroid gland and for aiding communication skills.