I would like to reply that this is not a normal behaviour.
I am not the type to give you a final conclusion on this matter, but the answer closest to the truth is a lack of minerals. Try increasing the amount of mineral/vitamins given to your chameleon, or change the brand. Personally I use the Miner-All Indoor from Sticky Toungue Farms with great success.
I had a few insidents way back with a chameleon eating dirt when I used a different blend from T-Rex.
The same often goes for panthers eating foliage, wich is not a behaviour that often occors in wild panthers.