Commonly, pure gold is sold by the ounce, whereas karat jewelry (kt.) is typically sold by the gram. To determine the cost of gold in a certain item of jewelry, we must convert ounces to grams.
31.1 grams equals one troy ounce. If one gram of pure gold costs $1300 per ounce in the metal markets, one gram costs $41.80 ($1300/31.1g). 11.6g is 58 percent of 20g. In other words, just 11.6 grams of that 20 gram necklace are pure gold. And, at $1300 per ounce (or $41.80 per gram), the gold in the necklace is worth $484.88 ($41.80 x 11.6 = $484.88).