What are the functions of the Third Eye Chakra?
This is the chakra that has to do with your insight, wisdom, discernment, and intuition. It’s where your willpower is balanced and your higher consciousness influences you. Spiritual experiences are processed through your third eye and it determines how you perceive your life and the people in it.
When it’s balanced, the third eye chakra brings you amazing insight and divine guidance. You may receive psychic flashes and prophetic dreams that you’re neither obsessed with, nor frightened by.
You’re stable in your own faith and spiritual beliefs, yet open-minded enough to listen to the beliefs and ideas of others. You’re calm and collected, not in competition with anyone - not even yourself. While you recognize you're a work in progress, you'll know how to push yourself to make the most of what you have. You have a good imagination and have no trouble planning for and visualizing what you want to manifest.
Where is the Third Eye Chakra?
This chakra is located between the eyes.  Because it’s located in the brow area, you might hear it called the brow chakra. 
What Color represents the Third Eye Chakra?
The Third-eye chakra resonates with all shades of purple.
The color purple has long been thought to be the color of royalty. In color healing, purple is believed to lift the spirits and help to combat sadness and despondency. Using healing crystals and precious gemstones in these hues will enhance energetic balance.
Symptoms of an Overactive Third Eye
It is way easy to become delusional when the third eye is out of balance. You take things to the extreme, creating your reality from what you perceive rather than what is actually happening.
You might blow things out of proportion and are easily swayed by mystics, psychics, and false prophets. You have trouble discerning fantasy from truth.
Using Amethyst and Lapis Lazuli will help you to restore balance to an overactive crown chakra. 
Symptoms of an Underactive Third Eye
When the third eye is underactive, you may feel uninspired, unmotivated, or that you’ve no sense of direction and purpose in life. Your intuition is either silent or you don’t trust it. You may even not believe in anything you can’t see in the physical world.
Spend time meditating to bring things back into balance. Your main goal is to find a way to let go of conditioning and fear. While danger is real, fear is imagined. Your subconscious self may be trying to reach you through dreams so keep a dream journal to take notes of anything that stands out.  Bright purple and dark blue stones are best to stimulate the third-eye chakra.
Work with Clear Quartz and Sodalite to stimulate your third eye chakra.