This swatch is seersucker, in Merino Magic 8-ply, shade 217, 4.5mm needles, 29 stitches cast on. I had trouble getting a photo that showed the diamonds and the colour well.
Apparently the word is originally from Persian “shīroshakar” and means “milk and sugar”. The derivative Hindi word “sīrsakar” is where the fabric name came from. I’m having a hard time reconciling that cloth with this knitting stitch though.
I found 19 patterns on Ravelry in a search for seersucker. Some were this stitch and some were a puckery, lengthwise oriented stitch that looks more like the fabric. In a greatly magnified kind of way.
I like this a lot. It looks great with ribbing in this hat, although I’m not sure how this would translate to the curves of a neckline. More swatching needed if I decide to use this stitch.