What's the story with Henna?

Back in my college days, I used to be what us pro's now call a kitchen beautician. I colored my own hair everything from black to purple and back to blonde so many times that no one was very surprised when I decided to become a professional hair stylist. Along my journey I found henna, and from what I remember it looked like mud, smelled even worse and left me with pinkish tangerine colored hair. Clients have asked me what I think of henna as a natural alternative to traditional hair color, and I have always been cautious. When asked recently I decided to do a little research, and what I learned surprised me. There are two types of henna on the market, pure henna, made from the actual plant and another type of henna that has metallic compounds. The metallic compounds can be anything from lead, silver nitrate, copper, nickel, or iron salts. This is very dangerous to your health and your hair. I remember seeing someone's hair smoke and nearly disintegrate when color was placed on previously hennaed hair in my early hairdressing days. The difficult thing about henna is that it is hard to tell the good from the bad stuff? I have seen it sold anywhere from Whole foods, to Indian groceries and even Lush. I guess if you insist on experimenting yourself, I would recommend reading the ingredients if they are listed, and if they are not don't buy it!