What’s on the van? – Moss agate

This week’s What’s on the van? comes from Monica Price, Assistant Curator of the Museum’s Mineralogical Collections.

This beautiful polished moss agate is about 8 cm across. It is one of 26 moss agate slices collected by John Middlemiss Luff, and given to the Museum in 1909. John Luff was a civil engineer who, as a young man in 1863, took a posting to the public works department in Bengal, India. Some of the finest moss agates come from India, and every one of the samples in John Luff’s collection is of the best quality.

Despite its name, moss agate doesn’t contain any plant material at all. The orange and brown ‘moss’ is made up of crystals of iron minerals, mostly goethite (iron hydroxide).  The crystals grew in a translucent white gel composed of silica (silicon dioxide) which solidified to form a mineral called chalcedony. Moss agates are found filling fractures and gas bubbles in volcanic lava. When the lava is eroded away by rivers or the sea, the hard lumps of moss agate survive, to be washed up as pebbles on river banks or beaches.

Moss agates take a beautiful polish and they have been prized as semi-precious gemstones for thousands of years. Brown moss agates are sometimes known as ‘mocha stone’. This has a connection with ‘mocha’ coffee beans, for both are named after the port of Mocha in Yemen, from which they were traditionally exported.