Meaning of the Quote

At the very first of the book Pobby and Dingan, there is the quote "The secret of an opal's color lies not in its substance but in its absences." which means the true meaning of opal in this book is related with something that is absent. Then what is the absence this quote is talking about?
I think it means the absence of trust, trust between the family.
In terms of the family, at first, all the family members except mother not belived Kellyanne's imaginary friends Pobby and Dingan. Father was just caring about an opal, which he never couldn't find. He was just caring about being rich. As soon as he lost Pobby and Dingan and worsely, he got arrested, his family became completely desperate. Kellyanne got sick and mother was crying whole day. That was because they didn't trust each others. Later, Ashomol went to the claim in order to find Pobby and Dingan, he started to believe the existence of them. He found himself calling Pobby and Dingan as if they were real. And he found the opal which is believed as Dingan's bellybutton. That was because he believed it. The true color of the opal turns in to real when the family members started to believe each other.