This is a very complicated subject, as "crystals" is not a very clear or concise subject.There are many types of minerals that come in crystal form and the knowledge of just which one yours is makes for another lecture in itself. Quartz varieties are usually fairly easily identified, although many have other minerals included in or with them. In the case of other inclusions or added attached minerals, care must be taken to have them identified so that no disastrous results will be encountered when using acid agents
Coloured crystals such as Amethyst, and believe it or not petrified wood, will bleach out if acids are used to clean them. Brazilian Agate will bleach by soaking it in too hot water. You live & learn, after you have ruined a specimen!
Clear Quartz crystals may be soaked in oxalic acid to remove iron stains if they are not intruded into the material. They may look as if they are on the outside, but repeated soakings prove that they are underneath the surface. First remove any outside clays and dirt with water and a stiff brush. Warm, but NOT HOT water helps the acid to activate a little faster. Neutralize the acid by rinsing in water to which baking soda has been added.
There has been quite good success in removing iron stains by soaking in household bleach. This only works if the stains are surface ones. And must never be used on coloured stones------it bleaches!
For a final rinse which will not leave smears on your crystals, you can use detergent. Harsh soaps must never be used as they can damage the surface of your specimen.
Soaking clear crystals in Denture cleaners is said to work with excellent results.
An important point to remember is that, if you are in any doubt as to the identity of your specimen, try each of the suggested cleaning methods on a poor scrap or a piece of quartz in a similar condition. This way you will not injure your good ones by experimenting on them before you are sure it will not do damage.
ALWAYS ADD ACID TO WATER!!
Remember that acids must be used with great care!
Always make sure that you have ventilation, and keep stored away from accidental spillage. Label very clearly and handle carefully at all times.
By June McKenzie