This is "A Young Man holding a Ring" by Jan van Eyck, painted around 1450. The subject wears a dark grey doublet with a red laced collar and chaperon, a fashionable headdress of the time. Notice how his fur-trimmed cuffs and collar are spotted - this is unusual in mid-fifteenth-century Netherlandish painting and might be made from lynx.
He holds a gold ring set with green and red cabochon stones, which could be a wedding ring or an advertisement for a goldsmith's trade. The background features blue and white stripes with rain clouds, including the words 'har las uber gan', which translates to 'Lord, let it pass over'. This is likely the sitter's motto, asking God to let trouble pass away.
Van Eyck was known for his subtle caricatures in portraiture, and here he's narrowed the man's shoulders and enlarged his nose to emphasize a distinctive feature. The artist would have followed several stages to create this painting, from preliminary sketches to finishing touches. You can even see some of these earlier designs using infrared radiation. Despite being over 500 years old, the painting is in very good condition.
A Young Man holding a Ring hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.