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* Revenue jumps 137 percent to $65.7 mln
* EBITDA falls about 8 pct to $18 mln
* Shares down marginally
By Karen Rebelo
March 3 (Reuters) - Precious stones miner Gemfields Plc's revenue for the first half more than doubled as demandfor its emeralds mined in Zambia grew and per-carat pricescontinued to rise.
Gemfields, which produces about a fifth of the world's roughemeralds, said revenue soared 137 percent to $65.7 million forthe six months ended Dec. 31.
Last month, Gemfields reported record revenue of $36.5million from its latest auction in Zambia.
The company saw an average price of $54 per carat in theauctions held in July last, while the average price hit a recordhigh of $59.31 in the auctions held in February.
Gemfields had been selling output from its flagship Kagemmine around the world until the Zambian government said lastApril that all emeralds mined in the country must be auctionedat home.
"Auction prices, despite the location switching to Lusaka,have been strong as demand for Zambian emeralds strengthens,"JPMorgan Cazenove analyst Alexander Mees wrote in a note toclients.
The company has two more auctions scheduled before the closeof its financial year on June 30.
Gemfields' core earnings or earnings before interest, tax,depreciation and amortization (EBITDA) fell about 8 percent to$18 million due to costs related to its luxury jewellery brandFabergé, and its Montepuez ruby deposit inMozambique.
"Gemfields is a company that's not just a mining company.We're building our inventory, we're building marketing andbuilding demand," Chief Executive Ian Harebottle told Reutersexplaining the developmental costs.
The London-listed miner has long held ambitions of creatinga stable world market for emeralds and polishing the gem's imagein the same way that Anglo American Plc -owned De Beershas championed diamonds from London's Bond Street to China.
"H1 FY2014 revenue was up significantly, due to the timingof auctions and an increase in average selling prices, but costsalso grew as marketing expenditures ramped up," analyst Meessaid.
Unit production costs rose to $0.85 per carat from $0.57 percarat a year earlier, the company said.
Production during the period fell about 28 percent to 10.4million carats of emerald and beryl.
Gemfields shares were marginally down from Friday's close at35.6825 pence at 1041 GMT on the London Stock Exchange onMonday.
