In the course of the havdalah ceremony, we sniff at besamim -- aromatic herbs, spices, or fruit (whole cloves are very popular). With the entry of each Shabbat, the soul of every Jew is uplifted by the presence of a neshamah yeteira, an additional spiritual dimension, a "Shabbat Soul." With the departure of Shabbat and the arrival of another mundane work week, this neshama yeteira departs, leaving behind a gloomy soul. According to kabbalah, of all the five senses, smell is the only one which impacts the soul. The fragrant smell of the besamim comforts and soothes the soul at its most difficult juncture of the week.
G‑d then gave Adam the wisdom to rub together two flint stonesDuring the seven days of creation, the world was illuminated by a great light which did not fade even during nighttime. As a result of Adam and Eve eating from the Tree of Knowledge, this light was hidden away immediately after the first Shabbat. For the very first time, the world was engulfed in darkness. A very frightened Adam exclaimed, "Woe is to me, for because of my sin the world has become dark!" G‑d then gave Adam the wisdom to rub together two flint stones, and when fire sprang forth Adam blessed G‑d for creating the light which emanates from fire.
We, too, recite this blessing every Saturday night to commemorate the creation of fire, and to thank G‑d for granting us its illuminating powers. It is also a timely blessing considering that for the past 24 hours we were forbidden from creating any form of fire.
It is customary to use a braided candle with multiple wicks. If such a candle isn't available, it is sufficient to use two candles (or matches, if need be) and join the two fires when reciting the blessing. This is because the blessing is in plural form -- He who created the "illuminations of fire." The plural "illuminations" is also an allusion to the various colors of fire.
After reciting the blessing on the candle, it is customary to look at one's fingernails. In order to recite the blessing on the candle, it must be sufficiently bright to be able to benefit from its light. If we can use its light to distinguish between the fingernails and the flesh above it, we know that the light is sufficient. Furthermore, nails are a symbol of blessing, for unlike the rest of the body, they never stop growing. Thus gazing at them is a good way to start a new week.
It is customary to hold the thumb concealed beneath the four fingers when gazing at the nails.
In some circles, women abstain from looking at their fingernails.