Agate is a Micro-crystaline form of quartz (tiny crystals, microscopic, not big, gemmy crystals), and characteristically is at least partially semi-translucent. It is hard, takes a very nice polish, is tight-grained and comes in an amazing array of colours.
Agate often exhibits banding and flow patterns. It is formed in pockets left in lava, which over time, have filled with deposited minerals.
In addition to Agate's natural range of colours, it also takes dyes well and can be had in a stunning array of fashion colours.