The hostrock for fine-banded greyish - white agates ont the Isle of Rhum is a tertiary basaltic rock with a low concentration of silica. It is unusual that agates occur in such silica poor rocks. Today it is forbidden to collect any agates on the Isle of Rhum. The isle of Rhum is a natural reserve for protecting red deer.
More informations about scottish agates are given in Dr.H.G. Macpherson´s book “agates” (published by the British Museum (Natural History) and the National Museums of Scotland).
You can find more informations about scottish agates on the following webpages:
Chris Harlows webpage about scottish agates
Nick Crawfords webpage scotsagate
H.Currie Minerals of Scotland