About half of the nickel mined each year comes from sulphide orebodies, for instance those in the Sudbury area of Ontario and the Kambalda area of Western Australia. The other half comes from deposits in which nickel leached from rock-forming silicate minerals has been concentrated by weathering.

There are huge nickel resources in the manganese nodules that litter ocean floors. In the 1960s and 1970s there was great optimism about deep sea mining, even though harvesting the nodules has been compared to using a straw to suck peas off the pavement of Fifth Avenue from the top of the Empire State Building. There are legal and environmental problems too, so the nodules will probably stay where they are for long while yet.

Nickel - symbol Ni, atomic number 28

For more information see the website of the Nickel Development Institute (www.nidi.org).

O is for Opal, best known in the precious varieties that flash brilliant colours. Opal as such is simply an amorphous or microcrystalline form of silica that contains water and solidified from a colloidal gel. It is very common in young volcanic areas, for instance as sinters around geysers and hot springs. Silica secreting organisms have contributed huge amounts of opal to sedimentary rocks.

Colloidal silica takes up water, and can retain a lot of it when it solidifies. Drying out leaves it full of minute cracks and cavities, which explains why opal is distinctly softer and less dense than quartz, quite easy to impregnate with dyes and other fluids, and prone to colour loss if it is not suitably treated. It is sensitive to pressure, shock, heat and cold, so great care has to be taken in setting precious opals and looking after them.

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