Also called metallurgical coal, it's a vital fuel used to make steel, the backbone of global infrastructure and of China's industrial boom.

The Steel Index's premium hard coking coal index has gained 90% in the past month. It's now the best-performing commodity of the year, The Financial Times noted, up 166%.

A supply squeeze in China has been the main driver of the rally, according to Alex Hacking, an analyst at Citi.

The Chinese government moved to reduce its excess coal capacity to boost the industry's profitability and curb pollution. It cut the number of days miners are permitted to work.

Also, shipments from Glencore, a metallurgical coal producer, were derailed in Australia last weekend, and this helped lift prices, according to RBC Capital Markets.