The saying "secrets transform into diamonds" suggests that 

hidden truths or undisclosed information, when brought to light or revealed, can ultimately result in something valuable or powerful.

This metaphor draws parallels between the formation of diamonds under intense pressure and heat, and the process by which concealed information can, through various circumstances, become something precious or impactful.

The exact phrasing "secrets transform into diamonds" is not found in the search results for novel titles containing "diamond" and "secret."

 

However, the quote is likely a metaphorical statement about secrets having value, and being revealed as something precious. The themes of secrets and diamonds are prominent in several works:

• The Famous Five series by Enid Blyton includes the novel Five on a Secret Trail, where the children likely uncover a secret that leads to an adventure involving a treasure hunt. In Five Go Off in a Caravan, the children's mother's past is hinted at, suggesting the possibility of a concealed past and hidden wealth.
• The Count of Monte Cristo by Alexandre Dumas involves secrets, hidden treasure, and the theme of delayed gratification. A fellow prisoner, Abbé Faria, educates Edmond Dantès and informs him of a hidden treasure on Monte Cristo. This aligns with the theme of secrets eventually yielding valuable outcomes. After escaping, Dantès finds the treasure and uses his wealth to exact revenge, while also performing acts of generosity, hinting at the potential for secrets to be used for both positive and negative purposes.
• The Diamond of Darkhold by Jeanne DuPrau features a secret discovered by Lina and Doon that could change their lives. This secret, a forgotten technology to harness the power of the sun, highlights the idea that concealed knowledge can lead to breakthroughs and new opportunities.
• The Diamond Secret (Love Inspired Suspense) by Lenora Worth involves a search for a legendary chocolate diamond and a dangerous man. This could potentially connect secrets and diamonds in the context of the story's suspenseful plot. 

In conclusion, while the exact phrase might not be present, the concept it evokes is found within the themes of secrets and diamonds in various novels like The Famous Five, The Count of Monte Cristo, and The Diamond of Darkhold.