I blipped a similar image two years ago when I described what you see as "a type of mastic exudate that once had medicinal uses".
Now I find that plum gum is once again poised for a curative role. A recent study has discovered that the substance can be used to deliver nano-particles that reduce inflammation but also that the gum itself has natural healing properties, thus providing a double benefit.
You can read about it here.
(Extras: Painted ladies are back, but even more exciting to see was one of the fritillaries right here in the lane, yards from the house. Which one is it? Silver-washed has been suggested.)
Update: the money's on a dark green fritillary Argynnis aglaja.
Update on update: Mollyblobs, for whose expertise I have the greatest respect, has opined that it's a female silver washed fritillary, Argynnis paphia