A treasured piece of moon rock showcased in a key Amsterdam museum is nothing but petrified wood, museum authorities said of a gift made to a former Dutch prime minister by a US envoy.

The exhibit at the Rijksmuseum, was originally gifted to Willem Drees in 1969 by then US ambassador William Middendorf as a souvenir of a pathbreaking trip by three US astronauts on July 20, 1969.

"When we received it, we insured it for 100,000 florins or 50,000 euros [$85,000] in today's money," said Xandra van Gelder, the editor-in-chief of the museum's internal magazine.

But she said that it was "hardly worth 50 euros," adding that museum authorities had been alerted by space and other experts.

This was confirmed by independent studies on the object, which is roughly the size of a matchbox.

"What is also strange is that in 1969, when Willem Drees was given this 'rock,' it was 11 years after he had stepped down as prime minister."

The US space agency NASA gifted pieces of the alleged moon rock to several countries.

The Rijksmusuem, one of the world's top museums, is better known for its vast collection of paintings by Rembrandt.

- AFP