It is from an herb, I think from Austrailia or something, it has exellent anti-bacterial qualities and is very soothing, etc. It also has a quality like menthol, but not as harsh. Gives a sort of mild tingling, refreshing effect, but not quite as toxic tasting as menthol. Seemed like the right thing to add.
I just found this article
Tea Tree Oil
Origin: Australia
Appearance: Pale to light yellow liquid having a warm spicy odor.
Aromatic Actions: cleansing, cooling, energizing, stimulating, uplifting
Blends With: clary sage, clove, geranium, lavender, lavandin, lemon, marjoram, nutmeg, pine, rosemary, ylang ylang, spice oils
Home Uses: abrasions, abscess, acne, antibacterial, antifungal, antiinfectious, anti-inflammatory, antiseptic, antiviral, athlete's foot, blisters, boils, bronchitis, candida, catarrh, chlamydia, cold sores, colds, cuts, cuts, dandruff, ear infections, febrifuge, fever, flu, herpes, influenza, insect bites, jock itch, mumps, oily skin, pneumonia, ringworm, shock, sinusitis, sunburns, thrush, tonsillitis, toothaches, vaginal infections, warts, wounds, yeast infections
How It's Made: The oil comes by steam or water distillation of the leaves.