The "Druids' Stone" ("La Pierre des Druids") typically named by local legend, is in fact a partially collapsed dolmen at Puy Pantout (Pantout Summit), due south of Treignac. Far from being a mere "rock outcrop," like Dolmen dit La Maison du Loup, the site, which rests at the terminus of a paved causeway (the final footpath up the hill - see pic), comprises a capstone, (typically) aligned along the summer solstice meridian, resting atop a chamber composed of four other massive stones, placed for a half-panoramic view to other nearby peaks. That the stone was imported is apparent while hiking up the track: no other granite cobbles or outcrops are visible anywhere off the path. Further, the natural fracture lines of the placed stones and the capstone do not align in either their horizontal or vertical planes, demonstrating that they were placed in position (rather than being exposed by weathering). Finally, there is no other grey granite strata visible anywhere else on the summit or along the many logging roads that expose bedrock right up to it.
The "druid" legend stems from two double-hand-sized querns carved into the capstone (see pic). These -- posited in local lore to be the offering table for blood sacrifice -- were likely used for grinding chestnuts or white acorns, both native to these upland copses. In summary, this site is typical of a substantial burial chamber, long since looted and re-used as a meeting place. See pic above for the full text and sources; I've translated the summary section here:
"The Pierre des Druides is certainly a megalith in the form of a dolmen, located in a remote place but visible from a distance, like a signal (until the 1960s the Puy Pantout was unforested, and covered with moors). The cavity one can enter beneath the supports could have served as a sepulture chamber... The inclination and the setting of the capstone indicates a monument connected to the cult of the sun. The presence of the basins, their aspect and disposition, the orientation of the table, and the situation of the whole, suggests their placement to hold or capture sacred water or blood from sacrifices."
The hiking trail is well-marked from Treignac, where a trail map is available from the Tourist Information Center on the Place de la Republic. It is a 3-hour round trip by foot from town, but it is possible to drive paved roads up to a trail head. Thereafter 4WD or mountain bikes can get you most of the way, but you will have to summit on foot. There are also marked trails to the summit from Allogne (to the east) and La Peuch (southwest). Access from Affieux itself is also possible, but more difficult.
The linked "Location Source" above is incorrect: that is a different site. The linked Panorama image is correct, but out-of-date: since then, the tourism board has placed explanatory placards (see pic) and scouts have cleaned up the site and cleared the field of view visible from the dolmen.
Something is not right. This message is just to keep things from messing up down the road