(Reuters) - Diamonds may be Tiffany & Co's (>> Tiffany & Co.) best friend as strong demand for the sparkling gems in China helped lift the U.S. jeweller's global sales in the latest quarter, leading it to raise its profit forecast for the year.

Diamonds have grown in popularity in China's 465 billion yuan (49 billion pounds) jewellery sector as affluent Chinese increasingly opt for diamond jewellery over gold as gifts for special occasions. Diamonds accounted for 30 percent of the sector this year, up from 25 percent in 2008, according to research firm Euromonitor.

"Traditionally the whole wedding jewellery segment was dominated by gold jewellery, but with Western influences the younger generations are more open to a diamond culture," said Mariana Kou, a Hong Kong-based investment analyst for CLSA.

There were fears this summer that luxury spending in China might slow as the economy there weakened, but Tiffany is the latest Western brand to report good sales there. Prada SpA (>> Prada S.p.A.) and Coach Inc (>> Coach, Inc.) recently posted big gains in the world's fastest-growing market for luxury goods.

Sales at stores open at least a year in Asia, except for Japan, rose 13 percent in the second quarter ended July 31, held up largely by China. That helped make up for some disappointing business in Tiffany's home market.

Tiffany could be set for an extra China sales boost with the local premiere of glitzy U.S. film "The Great Gatsby" at the end of August, analysts said.

Tiffany designed the jewellery for the film, which stars Leonardo DiCaprio and Carey Mulligan, and the jeweller's official page on China's Twitter-like Weibo microblog was packed with film-related material on Wednesday.

"Having specially designed Tiffany jewels in the film is like a sneak preview of the firm's work. Those cufflinks that Leonardo (DiCaprio) is wearing are definitely worth a second thought," one user said on Weibo.

QUIET IN AMERICA

But same-store sales were unchanged in the Americas, which is still Tiffany's biggest market. This suggests the firm may have faced the same summer pullback by U.S. shoppers that hit sales at chains from Saks Inc (>> Saks Inc) to Target Corp (>> Target Corporation).

"Business in the Americas is light," said Edward Jones analyst Brian Yarbrough. Tiffany continues to struggle with low-end jewellery sales, he added.

Tiffany executives told analysts on a conference call that tourists' purchases had helped business tick up at the Fifth Avenue flagship in New York, which generates about one-eighth of sales. Elsewhere, though, there was still reason to be prudent, they said.

"We are maintaining a cautious sales outlook for the Americas until we see solid evidence of an upturn," Chief Financial Officer Patrick McGuiness said.

Shares of Tiffany dipped 1 percent to $80.82 after it reported the results.

An Ipsos poll conducted for Reuters earlier this month found 35 percent of Americans planned to spend less on jewellery in the 2013 holiday season, while only 5 percent expected to spend more.

Tiffany said it still expected net sales worldwide to increase by a mid-single-digit percentage rate for the year, including the effect of the strong dollar.

The company has struggled to find the right mix of the expensive jewellery for which it is known and the more-affordable silver items, typically less than $500, that generate 25 percent of sales and comprise its most profitable category.

Still, the pickup in business outside the Americas, where Tiffany is focusing its expansion, reassured Wall Street that the jeweller's growth prospects remain good, Yarbrough said.

Sales in Asia outside Japan now account for about 22 percent of overall revenue, compared with 11 percent five years ago.

SALES MISS

The company, famed for its robin's egg blue boxes, said global sales rose 4.4 percent to $925.9 million in the second quarter, below the $941.4 million analysts were expecting, according to Thomson Reuters I/B/E/S. (Graphic: http://link.reuters.com/gup62v)

Sales growth would have been 8 percent if not for the strong U.S. dollar, which reduces the value of goods sold overseas.

Same-store sales climbed 5 percent, in line with estimates. Excluding currency fluctuations, they were up 7 percent in Europe and 8 percent in Japan.

Despite strong demand for high-end jewellery in Japan, overall sales there fell 14 percent because of the weak yen.

Second-quarter net income rose to $106.8 million, or 83 cents per share, from $91.8 million, or 72 cents per share, a year earlier.

Per-share profit beat the average Wall Street estimate by 9 cents, helped by lower pressure from diamond and gold costs.

Tiffany said price increases in some categories had not deterred shoppers.

The company now expects a profit of $3.50 to $3.60 per share for the full fiscal year, up 7 cents from its previous forecast range.

Last year, Tiffany's shares came under attack after it repeatedly lowered its forecasts. ($1 = 6.1217 Chinese yuan)

(Reporting by Phil Wahba in New York and Adam Jourdan in Shanghai; Editing by Lisa Von Ahn, John Wallace and Chris Gallagher)

By Phil Wahba and Adam Jourdan