Anyone who has been on an agate-hunting excursion knows that you rarely come across specimens that resemble agates found in museums, gift shops, or jewelry. The characteristic banding and speckles of quartz found on agates occur on the inside of the rock, only being exposed by cutting or by long term exposure to the elements. It is a rarity to find this distinctive layer of the mineral exposed naturally, so it is vital for any enthusiast to be able to recognize agate in the rough.
Understand how agates form. Agates are minerals (a type of quartz), not rocks, that are formed when groundwater containing silica finds its way into cracks in rocks and hardens in layers under the influence of surrounding pressure and temperature to form what we recognize as agates. The silicate minerals form like all crystals, in a specific pattern, which creates the banding. An agate's inner beauty is often hidden by an outer layer, referred to as the skin.
Consider the features of agate skin. This outer skin resembles the outer skin of rotted citrus fruit, bumpy and full of depressions. The skin varies in color; it can be red, bluish gray, green, or yellow. Holding a potential agate up to the light will show its translucence and weed out look-alikes.
Beware of rocks that resemble agates in the rough. Numerous varieties of chert, including jasper and flint, are cousins of the agate and often have the same outer coloring and texture as rough agates. To distinguish a look-alike from an authentic agate, hold it up to the light; if it is translucent, it is likely an authentic agate.
Chip potential agates with a sharp tool to see if you can locate any banding patterns. Rocks with conchoidal fractures (irregular breaks that do not follow a typical plane of separation) can be examined for banding.
Look for rough agates at surface level in places where it is plausible that water could leak into and collect, and from which gases could easily escape.
Research and then implement your knowledge of agate properties on a hunt. For a complete listing of the physical attributes of agates, visit http://www.mineralminers.com/html/agaminfo.htm. Agates are translucent, streak white, and have a waxy luster.