Dust

Dust, or more specifically, magic dust, refers to the dust from the Wood Between the Worlds used by Andrew Ketterley to make the Magic Rings that were the first link between Earth and Narnia

History
Even though the box was said to be atlantean, it is ultimately not known whether the dust inside originates from Atlantis or anywhere else. The dust he encountered he had inherited from his godmother, Mrs. Lefay. Although Lefay ordered Andrew to destroy the box which held the dust, he kept it after she died and began to try to study dust and its properties. Even before opening the box or knowing what was inside it, he described that he felt a tingling sensation of magic just from touching the box. This feeling was probably emitted from the dust itself and when Andrew did look inside, although it merely looked like sand, he knew immediately it was some higher force that he should avoid touching directly. After experimenting on guinea pigs he eventually managed to create yellow rings from the dust that magically transported any living being (as well as others that were through touch with another being linked to the dust) to the Wood Between the Worlds, which was first tested on Polly Plummer and Digory Kirke, as well as green rings that enabled anyone who touched them (or another living being linked to them through touch in the same way as the yellow rings) to escape from the Wood between the Worlds and enter other worlds by jumping into the corresponding pool in the Wood between the Worlds. Andrew Ketterley himself later also traveled using the magic rings, although he did so involuntarily. Other known travelers who have used these dust-based rings include Jadis from Charn as well as Frank the Cabby and Strawberry the horse of Earth.