Sometimes, it may happen that we do not know what a crystal is good for or how it can help us so I want to give my perspective on this.
Everything in the Universe is energy, frequency and resonance. Resonance means that similar energies share some similar properties or aspects which are interconnected. And usually these main aspects are reflected as colors too. Colors carry their specific energetic vibrations and aspects to them and affect us in such ways.
Crystals carry their colors and along with them, their energetic resonance. Our main energy chakra system also carries different colors and their resonance with different aspects of our life, consciousness and emotions.
A good and easy way to know what a crystal is good for, is to look at its color and see which chakra it resonates the most with. For instance, a red crystal will be good for the root chakra as the energy of red color carries similar aspects to those of this chakra (being grounded, survival and safety, material world, vitality, courage, action, life here grounded on Earth). In this example, a person with extreme fear of life and challenges, a person with low energy or a high level of poverty or dependance, will be greatly benefited from red crystals as these aspects are related to the root chakra and red color. A person who is immersed in confusion and lack of spiritual understanding would benefit from purple, violet and indigo crystals as these are the ones in resonance with the third eye and the intellect. These are important to see the truth behind lies and delusions and to find our path and purpose in life.
So the crystal color tells us which chakra it is good for. By knowing the properties of the main chakras, we can see what is missing in our life or what we can improve and then pick crystals which resonate with this aspect of our life.
However, one some few occasions, the color of the crystal is not the same of its aura. For example, I have seen my clear crystal quartz irradiating a purple-violet aura. I also saw a black obsidian irradiating violet and a light blue stone irradiating a strong indigo aura to my surprise. But in general, in most cases, the color of the crystal is the color of its aura.
If I want to see the color of the aura of the crystals, I put any black or white background and then see the crystal though it with the intention of seeing their aura. Then I relax and stare at the crystal until I perceive their colorful aura irradiating. It also gives me some insight on their energy and what their are good for.
Crystals irradiate a particular energy and frequency which connects to our aura or energy field helping us heal and balance different aspects of our life.
Important to remember to cleanse them from time to time when they feel heavy or when you wear them and shortly after feel drained. Then you know it is time for cleansing them and recharging them with new energy so that they keep helping and healing.