Maybe it was a cement/sand mix that was rinsed, dunked or lightly sprayed before the suggested 48 hours to set elapsed. Like maybe after only 24 hours. Wetted just enough to wash off the surface cement but leaving the sand grains exposed.

Interesting idea, Fleur. With a coarse enough sand grain that might work.

Your post got me to thinking, which is usually a dangerous thing.... But I had some "shavings" from the edges of my leaf castings and I poured some leftover concrete into a mold for a fairy door, then poured the shavings over the back of the mold I tapped with my fingers just a little, and left it to set. When I took it out of the mold a lot of the shavings fell off, put there are enough there that give it a grainy texture and look. Maybe that was how it was done?

We did this in grade school,
Materials:

paper cup
cementing solution (2 parts water to one part Epsom salt)
sand

Fill the paper cup half full of sand. Pack the sand with your hand.

Slowly add cementing solution until all of the sand is wet.

Put the cup in a warm place until the sand dries completely.

Carefully tear away the paper cup.
Carve........
Then we sprayed on something, and the little sculptures were rock hard. I don't remember what we sprayed........

Maybe this is what you saw
Marcia