• We said: “making one beer or soda can emits 170 grams of CO2… The average person gets through 120 cans in a year, which adds up to 0.2 tonnes of CO2” (17 November, p 34). That should have been 0.02 tonnes.
• The sparkling opal-like gems of sapphire and quartz that Tom Mossberg wants to make would, contrary to what we suggested, be more expensive than silicon gems (24 November, p 27). Also, it is the gems that are between 3 and 20 millimetres wide: the pattern elements are 60 nanometres.
