(CNN) -- Shortly before she and her family went into hiding, Anne Frank gave some of her toys to a friend for safekeeping -- a tin of marbles, a tea set and a book.
Anne had regularly played with Toosje Kupers on the streets of their Amsterdam neighborhood and tasked her friend with looking after her treasured possessions.
The next part of the story is well known -- the Frank family's suffocating experience of spending 25 months in cramped quarters, hiding from Jewish persecution by the Nazis. The world learned the harrowing tale in Anne's own words, written in her now widely read diary.
Now, some 70 years later, Frank's colorful marbles have been rediscovered and will go on display for the first time at a World War II exhibition in Rotterdam.
"Anne gave these to her friend for safe-keeping," Teresien da Silva, head of collections at the Anne Frank House museum, told CNN.
"She kept them for when Anne returned, but Anne did not return."
Kept in a box
Frank's account of hiding in Amsterdam during World War II, published posthumously in English as "Anne Frank: The Diary of a Young Girl," has moved millions of readers across the decades.
The young girl, with her parents and sister Margot, had moved into a concealed apartment on July 6, 1942 -- the "Secret Annexe" on the Prinsengracht Canal. Only a small circle of helpers knew of their whereabouts, and the family lived in fear of being discovered.
They were betrayed, arrested and deported in August 1944. Frank died of typhus in Bergen-Belsen just weeks before the Nazi concentration camp was liberated in 1945.
Kupers had kept the belongings, only finding them decades later when moving just over a year ago.
"She discovered she had the marbles somewhere at home in the attic," da Silva said. "She immediately contacted us. ... We were thrilled that the marbles survived and had been kept. She decided to give everything to the Anne Frank House -- the book, the tea set and the marbles."
The Anne Frank House Museum has already displayed the book, which Frank received for her 13th birthday, as well as the tea set, da Silva said.
"But we decided to present the marbles for the first time in Rotterdam," she said.
The marbles, alongside 99 other objects, make up the exhibition The Second World War in 100 Objects, which opens to the public this week. Dutch king Willem-Alexander will open the exhibition Tuesday.
Da Silva said Frank was one of many Jewish children who gave away their toys before going into hiding or being deported.
"For children during that time, marbles were a treasure. They worked very hard to win them," she said.
While the marbles are old, she said, they are in good condition.