The Third eye chakra or in sanskrit is called the "Ajna" meaning command and perceiving.
The third eye is related to the psychic senses, the sensing that we all have which is beyond or 'extra'sensory.
The most common area for the third eye to be located is between the eyebrows slightly above the bridge of the nose. Roughly in the middle of the head associated with the pineal gland.
The pineal gland affects our circadian rythyms, its sensitivity to light and its affect releasing DMT, the 'god' hormone.
Astrologically the ruling planet is Jupiter.
Some Gemstones that can help are Lapis, Amethyst, azurlite, quartz and sodalite.
Some imbalances in the third eye may cause one to have fantasies that appear more real than reality, indulgence in illusions. rejection of everything spiritual or beyond the usual, lack of clarity, or lack of guiding vision.