Such metals include lithium, niobium-tantalum, bismuth, antimony and rare earths. Such elements include boron, silicon, germanium, antimony, selenium, and tellurium. Boron, silicon, germanium, arsenic, antimony, and tellurium are commonly recognised as metalloids. Other examples include manganese, chromium, cobalt, nickel, copper, zinc, selenium, silver, antimony and thallium. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase its hardness. Silicon was used with arsenic, antimony and tellurium crystals. These metals were typically lead, antimony, bismuth or copper. The six commonly recognised metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Arsenic, antimony, and bismuth all have rhombohedral crystal structures. Together with nitrogen, arsenic, antimony, and bismuth, phosphorus is classified as a pnictogen. Chalcogens also bond with other pnictogens, such as arsenic, antimony, and bismuth. Other exceptions include antimony and bismuth. Emsley recognised four metalloids (germanium, arsenic, antimony, and tellurium); James et al. Binary compounds consisting of antimony or arsenic and a chalcogen.