Madagascar's interim president, who seized power in the Indian Ocean island nation last month, has unveiled a 300 kg (661 lb) gemstone that he said had been found in the presidential palace.

The dark boulder, streaked with shimmering green crystal, was shown on Tuesday evening inside the Ambohitsorohitra State Palace in the capital Antananarivo.

The stone, described as an "emerald in matrix", still needs expert analysis to determine the size and quality of the emerald embedded within.

"This is a national asset," said Colonel Michael Randrianirina, as he stood beside the find.

"It might be sold, and it will be up to the minister to explain the procedures that will be followed to increase the state's revenues," he said, promising "complete transparency".

He gave no details on how, when and where the stone was found, other than saying "upon arrival, we discovered this incredible national treasure" and "we do not know why it was brought here".

Mines Minister Carl Andriamparany called the gemstone a collector's dream. "An emerald with its natural matrix is rare," he said, adding that officials have found no record of a similar stone ever documented in Madagascar.

The government said proceeds from a sale would go into the state treasury.