Crystals are generally inert an perfectly safe, however, there are probably two aspects regarding their safety which your customers should be made aware of:

Children

The first relates to them as a potential source of danger to children; Because they are often so very beautiful they will inevitably, if the opportunity presents itself, go straight to the mouth of any young child. The most obvious hazard in this case is one of choking so a general warning is probably in order to the effect: 'Keep all crystals well out of the reach of small children'

Toxicicty

Although many of the crystals used in healing are based on some form of Quartz which is completely inert (quartz, citrine, amethyst etc.), others are not.  While in normal crystal healing use there is probably no hazard at all from these minerals and even if a child happen to swallow one, it would probably pass through their system without any ill effects. Nevertheless, you should be aware that there are crystals out there which under the right circumstances could present a harzard so it may be advisable to pass on gemtle warnings to your customers.

For your information, minerals which contain Lead and Mercury should be handled with care - examples of these are Galena (a beautiful but Lead based mineral, often bright metalic silver coloured crystal which is readily available) and Cinnabar (a dark redish brown ore of Mercury). The latter ore is reputed to have healing properties and is often made into jewellery (we've even seen trinket boxes made from it). We would advise that any ore of mercury should be handled very carefully indeed and not at all by children.

NB: We don't stock any crystal which we know to be toxic.