I should perhaps have entered this into the 'What is it?' challenge but it would probably take an expert in Myxomycetes to come up with the answer.
It's not elderberries. It's not caviar. This is a macro shot of a matchbox-sized piece of rotten wood that I removed from inside a damp tree stump on Hampstead Heath in London a few days ago. It was part of an area of soggy timber covered with tiny black pin heads which I reckoned were fungal in nature. I brought it home to try and identify. I couldn't - not without seeking help from the knowledgeable types on the UK Wildlife Fungus Forum.
These tiny black spheres (sporocarps), just under 1 millimetre in width, are the fruiting bodies of a slime mould, Metatrichia floriformis, which initially appears like a reddish-brown oilslick on the surface of the wood. A few days later this rash of sporocarps pops up and each shiny sphere will then burst open like a flower to expose a fluffy yellow mass which contains the spores. (I'm waiting in hope.)
Slime moulds are very interesting organisms but don't enjoy a good public image. They are no longer classified as fungi and are in fact closer to simple animal life forms, even to the extent of being able to move to engulf their food supply (rotting plant material or dung.) Their appearance can be off-putting: usually pallid or yellow, soft, slimy, gelatinous or like scrambled egg. One species that I have found is called Dog's Vomit since that is what is closely resembles. Fascinating!