Residues on stone artefacts: State of a scientific art

It is a startling experience to look down a microscope at a stone tool — a real Palaeolithic artefact, not a modern thing or a replicated copy — and see on its flint surface grubby brown-red stains that look the colour of old blood. Is a consensus emerging from the archaeological scientists as to just what traces of, especially, biological materials do survive on ancient stone surfaces, where they can be reliably characterized and identified?.