Tiffany & Co.’s third-quarter net income fell 30 percent, stung by a higher-than-expected tax rate, ongoing economic weakness and high precious metal and diamond costs.

The jewelry company’s results missed Wall Street’s expectations and it cut its full-year earnings forecast.

Shares dropped $5.58, or 8.7 percent, to $58.15 in premarket trading Thursday.

For the period ended Oct. 31, the company known for its blue boxes earned $63.2 million, or 49 cents per share. That’s down from $89.7 million, or 70 cents per share, a year earlier.

Analysts polled by FactSet forecast earnings of 63 cents per share.

Chairman and CEO Michael J. Kowalski said in a statement that Tiffany had expected its quarterly results would be affected by ongoing economic softness and tough year-ago comparisons. But he added that the retailer’s gross margin rate of 54.4 percent — down from 57.9 percent in the prior-year period — was weaker than expected and its tax rate was higher than expected. Gross margin, a key performance metric, is the amount of each dollar in revenue a company actually keeps.

While cautious about worldwide economic conditions, Kowalski said that the company anticipates results improving during the holiday season partly because of easier year-over-year sales comparisons but also because of new stores and new products.

The holiday season is critical for retailers, as it can make up to 40 percent of stores’ annual revenue.

Revenue increased 4 percent to $852.7 million from $821.8 million. Wall Street expected $858.8 million.

Sales rose 6 percent in Europe and 3 percent in the Americas region. Asia-Pacific sales climbed 2 percent, while Japan sales rose slightly. Tiffany said that its other sales jumped 73 percent as it converted five stores in the United Arab Emirates from independently-run distribution to company-run stores.

Tiffany now expects 2012 earnings of $3.20 to $3.40 per share. Its prior outlook was for earnings of $3.55 to $3.70 per share.

Analysts predict earnings of $3.59 per share.

Tiffany had 272 stores at quarter’s end.