Quartz was extremely important to the Irish. Quartz veins appear in more standing stone than I care to think about. There are a handfull of single quartz standing stones and Co Mayo has a load of quartz stone rows.

The quartz centre stone and/or outliers in the Cork circles seems to be peculiar to them. Surprisingly there isn't a stone circle made entirely from quartz stones like Duloe - although many are quartzite.

Perhaps the lack of an entirely quartz circle somehow defines it's specialness to the Irish. It does seem reserved for outliers or centre stones. I don't know of a circle that has any solid quartz orthostats.

The Irish name for quartz is grianchloch (scuse spelling), which means Sun Stone.