Henna is a plant, otherwise known as Lawsonia inermis. We also use the word "henna" to mean the dye we get from the plant. This henna dye has been used for thousands of years to dye hair, nails and skin.
Henna has been used since the bronze age throughout the eastern Mediterranean and Arabian peninsular. In Egypt, mummies have been discovered with henna on their hair and nails. Henna is also mentioned in the Bible and by the Islamic Prophet Muhammad, who is known to have used it. During the Islamic occupation of what is now Spain, and after it ended Spanish ladies often decorated their hands and nails with the red dye.
Jews, Muslims, Christians, Hindus and many other cultural and religious groups use henna designs for the celebration of marriage and often to celebrate the birth of a child.
The henna paste used in this decoration is made from a fine powder of crushed henna leaves. To this powder is then added some form of mildly acid substance like lemon or lime juice. Once this is left to sit, the paste is ready to use. In some cases sugar is added to change the consistency of the paste, and some essential oils containing monoterpene alcohols can also be added. An example of this kind of oil would be tea tree (Melaleuca alternifolia) and this helps the dye to stain more strongly.
The stain produced by henna is always some from of reddish brown. A light stain may only ever be orange, but most darken to red or brown. Henna is NEVER black.
Henna is never ever black. Some products are often marketed as "Black Henna" and may even contain an amount of henna itself but they also contain a chemical called Para-phenylendiamine or PPD. This is a chemical used in hair dye. It is an extremely strong sensitiser, which means the more often you come into contact with it the more likely it is that you will have a reaction to it.
PPD can cause blistering, open wounds, rashes and permanent scaring. It can also take anything from a couple of days to two weeks to start having an effect on you. Some research suggests that this chemical is also a carcinogen.
So always get natural red henna, never black.