Murder mystery author Agatha Christie took at least one secret to her grave when she died in 1976, at age 95: the location of her family jewels. Now, nearly 40 years later, diamond jewelry left to the author and her sister have turned up inside an old suitcase, and will be auctioned this week at Bonhams.

The suitcase was purchased at an estate sale at Greenway, Christie’s former home in Devon, UK, in 2006, by Jennifer Grant, a huge fan of the author. She paid a mere £100 ($160) for the traveling trunk, completely unawares of the treasures within. When Grant opened the suitcase for the first time, she found a mysterious safe box bolted to the base that she was unable to unlock.

“The strongbox made the trunk a great heavy thing, so it sat at the bottom of the stairs for years,” she recalled in a press release. “I almost didn’t want to open it because then the mystery would be over. When friends came round we would tip the trunk from one side to the other and listen to hear if anything rattled. If you were very quiet you could just about hear something light sliding inside.”

Four years and a crowbar later, the contents were finally unveiled: a pouch of gold coins, a three-stone diamond ring, and a diamond brooch.

In Christie’s biography, she wrote of the jewels that she and her sister Madge were to inherit from their mother: “My mother’s valuable jewelry consisted of ‘my diamond buckle’ and ‘my diamond engagement ring’…Madge was to have the diamond crescent, I was to have the diamond buckle.” A die-hard Christie fan, Grant had read the book. “I knew exactly what I was looking at,” she explained. “They matched the description exactly. I was nearly hyperventilating!”

The rediscovered jewels will be offered at Bonhams’ Knightsbridge Jewelry auction in London on October 8. The brooch is expected to fetch £6,000–8,000 ($9,600–12,800), while the ring carries a presale estimate of £3,000–5,000 ($4,800–8,000).