"Prehistoric Monuments and their Superstitions" is a chapter in Sir Edgar MacCullogh's 'Guernsey Folklore', which you can now read on the Internet Archive. There is also a chapter on "Natural Objects and their Superstitions". The book was edited and published in 1903, but much of the information was gathered many years before that.
Online mapping for Guernsey monuments. It's not brilliant - you don't seem to be able to filter by the age of the monument, which is a shame. And it won't give the grid reference. But it might help you a bit.
This consists of five enormous blocks of granite, laid horizontally on perpendicular piles, as large as their enormous covering. Around it, the remains of a circle of stones, of which the radius is thirty-three feet, and the centre of which coincides with the tomb. Mr Metivier says in his "Souvenirs Historiques de Guernesey" that this "Cercle de la Plain," in Norse Land Kretz, on this exposed elevation, could not fail to attract the attention of the Franks, Saxons, and Normans, and thus gave its name to the surrounding district.
In it were found bones, stone hatchets, hammers, skulls, limpet shells, etc., etc.
It is perhaps to this latter fact that we must attribute the idea which is entertained by the peasantry that hidden treasures, when discovered by a mortal, are transformed in appearance by the demon who guards them into worthless shells.