The meaning of the quote

Explain the meaning of this quote: "The secret of an opal's color lies not in its substance but in its absences." What does it mean to the characters in the novel and how does it relate to the plot? What do you think the opal symbolizes for the family and especially for Ashmol? Why is it important that it contains many colors and why are the absences important?

     This sentence is at the beginning of the book written by Australian Geographic, and it is really hard to answer because It does not show its meaning directly. The "secret"."opal's color","substance" and "absences” all have deeper meanings. In this sentence, the word secret means that the most important thing or the truly meaning. Then we can rewrite this sentence into “The most important part of an opal's color is in its absences not in its substance.” Opal is the core of the book, and it is the big background which surround the characters. Rex's dream was finding the opal. Opals are real to him. Ashmol likes those beautiful stones too. And there is even a big valuable opal on one of Kellyanne's imaginary friend Dingan's bellybutton. And this family is kind of be gathered together in Australia because of opal. Besides, at the beginning of the book, I remember that there is a plot about a teacher said to Ashmol, opals are magical and there are sprits in them. This plot creates a magical atmosphere for the opal, and let readers curious about how mystery the opals are. And in this book, opal also symbolizes dream, which is the support of people and like the guide light leads people walk straight towards their goals.

      Then I am going to talk about the substance and the absences in the sentence. I think the substance means the things that people can see and really exists, which symbolizes Ashmol's family members. They have different personalities and all of them are nice and sweet, so do the substance colors which are different and colorful. The absences means the things which we cannot see and maybe they do not exist at all, that symbolize Pobby and Dingan. They are imaginary friends of Kellyanne. They are invisible, and we cannot tell whether they are real exist or not actually. If you believe, they are exist; if you do not, they are not. But they brought huge influences to Ashmol's family, especially to Ashmol. They change Ashmol from punching air to show that he does not believe those imaginary things to a mature boy who can carry a heart with hope to find them in the deep mine during the midnight. They are Kellyanne’s imaginary friends, but they can change Ashmol and make him become more hopeful, positive and enthusiastic to face everything. They are like a gift. That is why they are really powerful and important. 

      In Pobby and Dingan, the main idea is that dream should be protected because they are valuable and important. 

      In my opinion, dreams can be separated in to two different types, one is your goals which is achievable and touchable, like opals, they are beautiful stones that Rex wants to get; one is your imagination, which is much more creative, and they are not real and even might not exist, like Pobby and Dingan, Kellyanne's imaginary friends. They are far from our real life, but they are still meaningful and important to people's certain period of lifetime. However, as I just mention, Pobby and Dingan are the absences of opal's colors. They are important and mystery and what the lies are telling. They are the other form of dreams which contains more of our hope and impossible, for it has endless possibility. No one could tell how they look like and why people have those ridiculous imaginations. But they are the truly meaning of a dream.