In the myths, there is often one of the nine or eighteen maidens who is different. There is a mother, or father figure, sometimes in addition to the maidens themselves. In Scotland you hear that the nine maidens worshipped an older version of the Cailleach/Bride called the shining one. The extra stone would maybe represent the shining one and the nine or eighteen maidens who worshipped there?

White cows were sacrificial animals. Special breed of cow of which only a few herds remain now. The true celtic cattle, not the hairy highland cow as advertised on a million shortbread tins. Little white quartz stones were used by poorer people into fairly recent times as an alternative sacrifice to silver at wells too. Some white cow stones often have stories of a white cow hide full of treasure hidden under the stones, but when people tip the stone over, next day it is said to be upright again. Some white cow stones are said to be cows the Cailleach turned into stone here in Scotland too.

Lots of connections if you let your mind wander.