CARACAS--Venezuela, facing default fears amid an economic crisis and falling oil prices, will use diamonds as well as other precious stones and metals stored in its central-bank vaults to boost international reserves, the central bank said Thursday.

It will also incorporate easy-to-convert foreign currencies into its reserves, the bank said in a statement explaining changes to a law regulating its activities. Venezuela's international reserves have dropped 28% in the last three years to $21.7 billion.

The country is likely to start holding some reserves in Chinese Yuan, a central bank official familiar with the matter said.

It is the latest effort by President Nicolás Maduro--whose approval rating hit a record low of 24.5%, according to a poll released this week--to try and salvage the oil-rich, but cash-poor economy. The country is facing crippling dollar shortages and scarcity of imported food and consumer goods. Its local currency, the bolívar, is in free fall, reaching new lows almost daily.

But analysts said the new accounting methods for international reserves would do little to build trust among investors, who currently charge Venezuela one of the highest borrowing rates in the developing world.

"It looks like a cosmetic boost to the reserves which doesn't improve investor confidence and doesn't improve reserve liquidity," said Siobhan Morden, head of Latin America strategy at Jefferies in New York. "This is a strategy of counting pennies to see how long they have."

Venezuela holds more than 70% of its reserves in gold bullion. Having a small portion of the reserves in cash has been a concern for investors who worry about the country's ability to honor debt payments. Venezuela's benchmark 2027 sovereign bond offered a yield of more than 19% Thursday.

Mr. Maduro this week authorized a 20% cut in government spending in light of the more than 30% drop in the price of oil, which Venezuela relies on for 96% of its dollar income. Finance Minister Rodolfo Marco is in China this week seeking financing and will next head to allied countries like Russia and Iran, Mr. Maduro said.

Last month, Venezuela used a $4 billion loan from China, funds that are typically earmarked for infrastructure investments, to beef up its reserves. But reserves since have declined again by $1.8 billion.

"We have a special plan to strengthen our reserves," the president said in a speech Tuesday. "They're going to rise further in the upcoming weeks and beginning in January and February," he said, promising to open up the country's access to "freely convertible currencies."

But those measures will only buy Venezuela time without changes to its heavily interventionist economy and simplifying rigid currency controls that maintain three different exchange rates for the dollar and deter investment, analysts said.

Barclays estimates that Venezuela's economy will be an outlier in the region this year, shrinking by 4.4%, followed by contractions of 6.2% and 1.5% in 2015 and 2016, respectively.

Write to Kejal Vyas at kejal.vyas@wsj.com