By Adria Calatayud

Shares in Karelian Diamond Resources PLC fell Wednesday after the company said a long-running confidentiality agreement with Rio Tinto PLC lapsed as the companies haven't entered into a further extension.

Under the agreement, Rio Tinto disclosed to Karelian Diamonds confidential information relating to exploration in Finland in exchange for the option to earn a 51% interest in any project identified, Karelian said.

The agreement expired on June 30 after nearly a decade, Karelian said.

Karelian, a U.K.-listed, Finland-focused diamond-exploration company, said it is considering the best means to progress its exploration program.

Shares in Karelian at 1413 GMT were down 8.8% at 2.60 pence.

Write to Adria Calatayud at adria.calatayud@dowjones.com