An 8th-century tortoiseshell vanity box, said to be a gift from the Tang Dynasty palace to Japan’s Emperor Shomu, will be auctioned in Hong Kong and may fetch more than HK$40 million ($5 million) for its rarity, said host Sotheby’s.

The octagonal box, measuring 35.6 centimeters across and embedded with mother-of-pearl and amber in shapes of flowers, is the highlight of Sotheby’s planned sale of antiques, gems and paintings on April 8, the first of its biannual auctions in the city this year that set benchmark prices for Asia’s art market.

Art-auction companies such as Sotheby’s and rival Christie’s International have been missing estimates at sales worldwide after the latest round of the global credit crisis began in the fourth quarter, slashing wealth and curbing art purchases. In Hong Kong, the second of Sotheby’s auctions tallied HK$1.1 billion in October, missing presale estimates by half, compared with the HK$1.77 billion total of its April sale.

Nicolas Chow, Sotheby’s Hong Kong-based head of Chinese ceramics and works of art, said he arrived at the box’s HK$40 million estimate after revising down an earlier figure by about 30 percent because of falling asset prices worldwide.

“This object speaks for itself,” said Chow, 34, in a telephone interview. “It’s truly exceptional.”

Most Tang Dynasty artifacts are exhumed, said Chow. Not so the tortoiseshell box, which is valuable also because it had stayed above ground, he said. The item, consigned to Sotheby’s by an undisclosed Japanese individual, had once resided at the repository of the Todaiji, or Great Eastern Temple, in the southern Japanese prefecture of Nara, Chow said.

Todaiji, founded by Emperor Shomu (reign: 724-749), was his government’s temple and received most of his cherished personal belongings after he died, donated by the Empress Dowager Komyo as a sign of her devotion, according to a Sotheby’s statement.

The box is one of three identical items, one still at the Todaiji repository and the other at the Museum Yamato Bunkakan, also in Nara, Chow said.