Bottles left in direct sunlight for prolonged periods of time (years and years), will change from to clear to an amethyst tint. This color is called sun-colored amethyst is quite attractive.

Some dealers will irradiate bottles to quicken the process and deepen the color. Clear bottles, especially straight-sided Cokes and Pepsi, will turn a medium to deep purple. This color is not natural and not rare, so buyer beware.

The same process is done to green hobbleskirt Cokes, and the result is an amber hobbleskirt. While attractive and novel, it is also not rare and not natural. So don't be lead on by stories that it is a rare one-of-a-kind prototype bottle.

For additional info on irradiated bottles, see the April 2002 issue of the Raleigh Bottle Club newsletter.

The preceeding was part of an online article on antiquebottles.com

If it looks "too" purple, it probably is. Irradiating a bottle is no different than painting an antique piece of furniture. It decreases its value and just looks plain rediculous. Of course, if you like the color and the seller is being honest about how it turned purple, bid till your heart is content.