Study of a woman’s hands, Leonardo da Vinci; black chalk and metalpoint on paper, roughly 8 x 6 inches (21 x 15 cm). Original is in the Royal Collection Trust in the UK; their website has both zoomable and downloadable versions of the image. There is also a version on Wikimedia Commons.
The drawing is actually not of a single pair of hands, but is two drawings of crossed hands, with one hand emphasized in each version.
When the description says “metalpoint”, the most likely actual medium is sliverpoint. Prior to the discovery of graphite, artists would ordinarily draw with charcoal, chalks, ink or metalpoint (for the moment, leaving aside printmaking). Metalpoint, though expensive, was preferred for the most delicate, exacting drawings.
In silverpoint, the artist draws with a thin silver wire, arranged in some kind of holder, on specially prepared paper. Over time, the silver lines oxidize to a warmer and more visible — but still delicate — line.
I’ve had the pleasure of seeing some master silverpoint drawings (though not this one), and it’s difficult to convey their subtlety in photographs. The only linework I’ve seen that might be comparable is in etching, whch is done with a needle.