At a glance, this apparent mineral inclusion appears to be calcite, judging by its silvery-whitish color and rhomboid form. Upon closer inspection, however, one may perceive that there is a small opening on the quartz surface, indicating that it is actually a hollow rhombohedral cavity with traces of a reddish clay-like material near the opening.
This type of cavity is known as an "inclusion void" and the example shown in the photo is the natural cast of a calcite mineral that dissolved away during the formative phase of the crystal, leaving a perfect impression of itself inside the optically clear quartz. Often an inclusion void may be entirely enclosed within the quartz host, making positive identification difficult.
Whether an actual mineral inclusion exists within the cavity, or if it is merely an empty void in the form of the mineral that was formerly present, may be difficult to determine. In this particular case the mineral cast is near to the surface of the quartz crystal and the opening to the hollow interior is visible, a clear indication of an inclusion void. "inclusion voids" -Inclusions in Quartz