This is a most unusual specimen and not easily explainable at first glance. It seems to show a fragment of green volcanic ash fixed on the inside of an agate nodule/amygdale.  How can this be? This was not found as a complete nodule but only as a fragment.
In the Devonian, 380 million years ago, this area must have been a very violent place with recurrent volcanic activity and earthquakes. Having seen areas of exposed rock in the Dunure area and seen what I can only describe as volcanic pipe structures within the basalt occasionally lined with crystals and agate, I would suggest this piece of ash ended up where it did by falling into one of these pipe structures. Further agate was then formed around it eventually enclosing it completely over a period of time. This is my best explanation of this unusual specimen.