This one took some last minute hunting . . . but thank Een I embarked on this quest; this ingredient turned out be rather essential for the activity I led last week in class.
Stones from the River Wink are colourful, as you can see. They tended to sink to the bottom of most of the charm bottles we made in class last wee. Thankfully, I remembered to bring a healthy quantity of goop to the class, so some of my clever apprentices were able to suspend the stones in their bottles, resulting in a very magical result.