After seeing the Gift's from the Thief Con, and the latest one made by Squight...my fingers started twitching to make my own.
I decided to ignore the artist interpretation on the new edition paperbacks, and just go to the source.
Descriptions of the stone include:
"Still caught in my palm after a night in the river was the poor, plain, gray-and-white spotted stone--Hamiathe's Gift."
"The magus flipped it over so that he could see the lettering carved there, the four symbols of Hephestia's ancient name.
'But it's just a plain gray rock,' said Sophos."
"It was a rounded oval and just the weight, I thought, to go in a slingshot. But I looked closer at the letters carved in the side of it and saw the sun glint off something blue at the bottom of the carving.
'It's a sapphire,' I said, "at least part of it is.' I peered down the hole bored from top to bottom, then flipped it over and, looking closely, could see where the water had worn the stone smooth and uncovered a few blue flecks of the gem inside."
"The sapphire hidden in the stone caught the light, and the carved letters seemed to hover, bright blue, in the air."
Ok, so based on those, I wanted to have a small, gray, speckled, rounded oval stone that at first glance looks boring until you notice the flecks of sapphire on the outside and the sapphire uncovered by the four symbols of Hephestia's name. It also needs a hole bored from top to bottom to thread a leather thong through. I didn't want a flattish rock like the one on the book cover, since Gen likened the gift to slingshot ammo....which to me implies a rounder stone.
After some experimenting with imbedding a blue glass gem inside a round "stone" made from clay and then carving through the clay to reveal the gem (didn't work...the gem would have needed light shining through to make the blue show up), and then with mixing black sand into clay to get a speckled effect (didn't work...upon baking, the "art sand" turned purple).....I ended up using a light shade of gray clay, rolled in real sand from my driveway (sifted to keep out larger chunks).
Mini experimental Gifts, post baked:

I added some minor accents of blue glitter on the outside (rolled the ball of clay between my palms to make the gravel and glitter imbed in the clay nicely) and then carved four symbols onto one side (I used the ones from the book cover for inspiration). I used a metal skewer to bore a hole straight though the "stone" making sure to keep the rounded oval intact.
After I baked the clay, I used acrylic paint in "ice blue metallic" to very carefully fill in the carved letters
A light coat of spray polyurethane to give it a slight river-polished look, and it was done!


(It's grayer in real life, not as tan as the pics show)
So...that's that.