There is something about combing the beach & noticing the change of texture & colour in the sand. A fragment of a sea urchin shell might give you a clue, but if you are looking with eagle eyes you will notice the crushed and broken tiny dark purple spines of the urchin combined with sand & the other broken conchology that create this texture & colour.
I have many sea urchin quills & shells in all shapes, colours & sizes. The quills I sell at The Society inc. are larger ones that vary in all shades of purples & a little bit of green with white stripes. When in your hand the clink together and sound like chimes. They have a soft fired, but not glazed, porcelain texture.
I was shooting at the Macleay Museum in the back room, in the back drawers (yes, it was as romantic as it sounds) with my brother/photographer extraordinaire & this is what we found!