Obviously, these were dark speckled lunar bluish "moon stones" and were chosen for being similar to the starry night through their speckled appearance. They were used for lunar and eclipse cycle calculations.
In this same Preseli area of Wales, I have been able to decipher Carn Arthur, Wales, Pembrokeshire, as hermetic astronomy, "as above, so below", showing that Carn Arthur marks the stars from Cancer to Cepheus and Pegasus, which corresponds roughly to the stars extending from the Spring Equinox to the Winter Solstice in ca. 4000 B.C. Always remembering that the ancients may not have had a 1-to-1 correspondence with our current stellar constellations, but close to them, Carn Arthur shows Canis Major and perhaps Canis Minor as a cat and puppy dog, Cancer as a woman, Gemini as two chicks (chickens), Orion as two persons with some kind of relationship to fertility, perhaps showing the birth of an infant and older children's faces, Lepus as a face of some kind, perhaps the mother-in-law (:-)), Auriga as a duck, Taurus as an extended serpent, Pisces as a fish, the Pleiades as baby ducks (Latvian Pīlites = baby ducks = Pleiades), Perseus as a head, Andromeda as a larger head, Cassiopeia as the "ribbon" in the hair of that head, Cepheus as a head, torso and leg from the side and back, and Pegasus (the Great Square) as a head.