Jeez, you guys need to do your research on this 'string theory'.
See, the purple string (and any other color, really) has been cruelly injected with dye...seems like purple is the most popular color now but blue and pink is really taking off around here. The glowing string has been genetically engineered, at first it was used for scientific purposes but now people just want glowing string because they think it looks cool and who cares about the possible future impact of genetically engineered string, right? Before buying ANY kind of string, make sure you know what string NATURALLY looks like.
Here is some natural string just so you can be sure you're getting unaltered string. Ask about where the rocks come from too, because there have been a few people duped about the authenticity of the rock...make sure it's not a cheap plastic imitation!