Cup shaped, inner fertile surface smooth, scarlet to deep red, outer surface whitish to pinkish to ochreaceous, attached to the substrate by a thick stem. On microscopic examination the tiny hairs on the outside of the cup are straightish.  Spore print white, spores 9-12μm wide; note there is a size overlap with Scarlet Elfcup). 

The Scarlet Elfcup, Sarcoscypha austriaca is almost identical

microscopy

Colour and size are unreliable features; microscopic examination of fruiting bodies is needed. Either submit micro-photos showing key features, or confirmation that it has been identified from a specimen by an expert.  If you haven't done this, submit as the genus Sarcoscypha and note habitat and substrate.

Grows in scrub or woodland on rotting mossy wood fragments in very damp to wet conditions.

There is more information on the First Nature webpage for Ruby Elfcup