The banks of the Tay - famous for agate-findings - are exploited. It is very difficult to find a collectable agate at this locations today. In this region collectors were able to find (besides the common colours and patterns) agates of the “sagenitic type” ( forming pseudomorphs after formerly existing needle-like minerals).
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