PORPHYRY is one of the strongest and most durable rocks in the world. It is a type of ‘igneous’ rock that has formed from cooled magma (molten rock) and is characterised by larger crystals dispersed within a finer-grained matrix.
The term ‘porphyry’ is from Greek and means ‘purple’. Purple was the colour of royalty, and the ‘Imperial Porphyry’ was a deep purple which was prized for various monuments and building projects in Imperial Rome. In fact, porphyry comes in a wide range of colours from blues and greys, through to greens, browns, reds and purples. This variety of colour – together with its inherent strength – is one of the reasons why porphyry has been so popular in Italy – particularly in cities like Verona, Venice, Florence and Rome. There are Porphyry monuments in Rome that are over two thousand years old and still look just as impressive today as the day they were built.
See the full Wikipedia entry for Porphyry here http://en.wikipedia.org/wiki/Porphyry_(geology)