Pebble

This description comes from my friend R, a geologist.

Disc-shaped pebble of a red slate/phyllite (probably iron rich) with
two quartz veins, which because they are harder than the slate stand
slightly proud of the phyllite. The earlier quartz vein is the thin
one (it has been repeatedly displaced by tiny movements along the
cleavage plains in the phyllite). The thicker quartz vein (not
displaced) is the younger but, still much older than the date at
which the pebble was formed.

For R