St. Modan was a 6th century Celtic monk who established a religious center in the Ruel River valley, home to Kildalvan and to the early ancestors of Joel Campbell. (See previous posts on Kildalvan) Kildalvan apparently had its own chapel ("Kil" means church) but it was likely part of St. Modan's religious community as it is just a few miles from where "Kilmodan" (the church of St. Modan) was located.
Recently, a spring was rediscovered that appeared on very early maps as "St. Modan's Well." You can read more about the discovery at the Faith in Cowal website.
The coordinates of the spring are approximately: 56.011412 -5.205267
Kildalvan was located here: 56.054845, -5.183572
The article at FaithInCowal.org suggests that quartz pebbles in the pool were "placed there most probably by long ago pilgrims as they said a prayer." As I have written before, quartz is ubiquitous in this area. A huge piece is embedded in the ruins of an ancient structure at Kildalvan.
Could Joel's ancestors have visited this holy spring, known as "St Modan's," and brought some quartz with them to place in the pool?