The Vermeer portrait is an enigma: we don't know who it's of, we don't know when it was painted, and we don't know what Vermeer would have called it. It's one of the most compelling portraits of all time.

It's been called the Mona Lisa of the North: and there is something of that mysterious effect in the slight sfumato of the mouth. But this girl is not composed and knowing as La Gioconda is; she's young, possibly innocent, and her mouth is half-open in an appealing or almost questioning look.

I can't tell how old she is: fourteen, perhaps sixteen, or maybe grown up. The rest of the face doesn't tell us much; it's the delicate budding of those lips that makes her seem so young, I think. She's standing facing to the left, with her head turned towards us. She's wearing a rich golden-coloured robe with a white collar, and a blue turban topped with yellow, falling down behind her. The painting is also called the Girl with Turban. That's certainly what dominates the space of the scene.

But the pearl earring: your eye is drawn to it. Her coral lips, her limpid eyes, and then that earring, hidden in the shadows away from the strong illumination on the left. It's an example of something of a Vermeer characteristic: the almost metaphysical or hypnotic power of a bauble, which when you look into it reflects the whole scene, or world, or captures the people and voices and lives around her.

The background is just black. The flesh, the gold, the blue, the white of her eyes, swim out of nothing at you.

The pretty model appears in another Vermeer, the allegorical The Art of Painting, standing robed all in blue, crowned with leaves and fruits, holding symbols of music and literature as the artist himself visibly paints her. André Malraux believed she was Vermeer's younger daughter Maria. Modern critics don't think there's any evidence for this, and don't think the dates fit, though no-one actually knows the dates. Around 1665 plus or minus five, but that sort of uncertainty covers too much of Vermeer's short career to be meaningful.

It's 46.5 cm x 40 cm, oil on canvas, signed, and in pretty good condition, very little disfiguring retouching. It was bought at Amsterdam in 1882 by one A.A. des Tombe for 2 florins and 30 stuivers, and without even checking that's got to be a ridiculously small amount for such a masterpiece: he presented it to the Mauritshuis, the royal gallery in The Hague, in 1903.

A rather unusual love story is the 1994 opera The Second Mrs Kong, with music by Sir Harrison Birtwistle and words by Russell Hoban, where the girl with the pearl earring's partner is King Kong. It was commissioned by Glyndebourne. Hoban gives her the name Pearl; and it also features Anubis, Orpheus, and Vermeer.