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The insurers say that - in an area where form and material is often of secondary importance to the idea expressed - the key is to protect against the loss of the certificate of authenticity provided by the artist.

"Since a piece of paper is often the only document essentially giving value to a work of conceptual art, we wanted to find a way to protect our clients' investments even if something happens to their certificate," said Jonathan Crystal, executive vice president of Crystal & Co.

Previously, this reference to lost or damaged certificates of ownership has not been spelled out in the small print of fine art insurance policies.

Ron Fiamma, global head of private collections for AIG Private Client Group, said the new coverage reflects a growing need as the art market embraces more challenging contemporary works. "Conceptual art collecting has increased in recent years, and as a result we have fielded more questions about policy contract coverage," he said.