Our chakra system works by taking in energy (in the form of experiences) from our environment, processing it, and then giving it out again. The chakras represent our lessons, with each chakra vibrating within its own frequency or spectrum of potential learning. For example, we process information through our base chakra to learn about issues relating to survival and being on the planet, how supported we are in life, our stability and our roots. If someone is in the process of moving, or are currently without a home, or their fundamental needs are not being met in some way, then you would find their base chakra was affected in some way. The heart chakra is how we learn about compassion, forgiveness, being more kind, loving and about how we are connected to others. If someone is going through a break-up, is trying to heal a rift with someone, or is attempting to move from a more self-centred perspective to a more open-hearted one then the heart chakra would also be energetically affected in some way.
When we work with crystals on someones chakra system (or within their energy field) we are not magically 'fixing' them or absolving them of responsibility in learning their lessons. Instead what we are doing is helping them to move through these lessons a bit quicker and with more grace. By putting a crystal on the base chakra this can help someone feel more supported and stable, or with a crystal on the heart chakra, this may help them release an old emotional energetic block (they may shed a few tears) that has prevented them being more loving with themselves and others.
Each of the chakras is a different colour; red for the base chakra, orange for the sacral, yellow for the solar plexus, and so on up to violet for the crown chakra. What these colours are representing are the overall vibration of each chakra. The base chakra is red overall because it is vibrating at a slower rate than the crown chakra which is violet and vibrating at a much faster frequency. Each of our chakras therefore has its own distinct vibration. This means that, if our chakras need some help, they can be nourished or be helped back into balance, by a crystal of a corresponding healing vibration. For example, if a chakra needs energising then a crystal such as Clear Quartz or Citrine (both have energising properties) could help. If its needs clearing then perhaps a piece of Snowy Quartz (a clearing energy) is appropriate.
You may find online or in shops crystal packs called 'chakra sets', within which there is a different crystal for each chakra. In principle this makes sense. Each crystal can have an appropriate vibration (which is linked in this instance to its colour) for the particular chakra it is matched up to in the set. However, there is a problem with this one energy suits all approach- which is that each of us is different energetically. Without tuning into a chakra, how do you know which energy it needs? Each of us also can change day to day, so a crystal that may be right for my heart chakra one day, could be too much, or not enough on another day. For example, today rose quartz may be fine on my heart chakra and help me open up and find greater balance on that level. However, tomorrow I may find childhood pain surfacing, and need something that can address wounds on a more deeper or more spiritual level (maybe morganite, another pink crystal which can work at a finer level). Alternatively, if I'm feeing fragile or have just been ill, Rose Quartz could be too much for my heart chakra, and I may need something much gentler like Mangano Calcite (a much softer baby-pink crystal with a very gentle energy). Or maybe I don't need a pink crystal, but a green one with a more balancing energy like Aventurine or Prehnite. On perhaps I don't need a 'traditional' heart colour at all, but an energy like Amber, which is very soothing, or Turquoise, which is strengthening and protective and can help address issues on a more profound level.
There is therefore not 'one size fits all' for any chakra, which is why chakra sets could actually end up counter-productive energetically. When a healer tunes in the most important thing is that they are able to ascertain energetically what a chakra, or another part of the subtle anatomy needs, and can then choose a crystal with a vibration to match. This is why if working with crystals its so important to have a serious and in depth training (of a minimum of two years).
If someone puts the wrong vibration on a chakra it may 'hurt' someone energetically. For example, for the higher chakras, you need crystals with at least a medium to high vibration. If a crystal with a denser vibration, is placed on the third eye or crown you could give someone a headache, or worse. Similarly if a too high vibration is placed on a lower chakra, you could unground them and space them out. Some chakras are much more sensitive than others, so the heart chakra and third eye for example, need to be treated with more care. Many online articles and books recommended crystals for the heart chakra merely because they are 'green' (a traditional heart colour) and therefore presumed okay for it. But actually they may be totally inappropriate and could 'hurt' someone energetically. Malachite is much too dense for the heart for example although it is green. Amazonite is another green crystal that is a bit too dense in energy. Rhodonite is another heart coloured crystal, being pink and black, but it has a grounding energy and is also much too dense to place directly on the heart chakra.
Choosing the correct crystals for someone is also related to someones overall energetic make-up and development. If someone is on a conscious path of self-development and working as a healer they are probably going to benefit from the higher vibrational or more profound crystals whereas someone who has never had healing work before could benefit from something more straightforward in energy. For the latter, if the crystals used were too high or profound vibrationally this would be inappropriate, forcing in too much light and possibly giving them awareness or insights before they are ready or able to process them.
Ultimately, for crystals to be truly effective, just placement on the chakras alone is not enough to make them potent. This is a good thing. For most people, even if a crystal is the wrong vibration for a chakra, there will be not too much harm done. The missing ingredient is the healer being able to channel a higher energy into the crystals. It is this energy which really 'turns up the power volume'; the crystals take and shape and focus this energy dependant on their own frequencies into the human energy field. And the energy that someone can channel is also related to their own self-development; almost like a cosmic fail-safe! We can only go so far working with the mineral kingdom until we have also begun to master ourselves. It is only when the crystal student is ready that the teacher crystal will appear!