Instability continued to roil China’s stock markets on Tuesday in spite of new pledges of support from the government. Shanghai’s main share index fell as much as 5 percent as trading opened on Tuesday, adding to an 8.5 percent plunge on Monday that was the biggest one-day decline since 2007 and which brought an end to several weeks of relatively calm movements. Stocks seesawed throughout the day, rising at one point by as much as 1 percent, and ended Tuesday down 1.7 percent. The precarious display on China’s bourses has shaken global financial markets. Prices for metals like copper, of which China is a major buyer, fell to six-year lows during the past two days. Oil has also slumped, trading closer to the six-year lows it reached this spring. […]