I think you'd want to avoid removing amulets as this might result in hostility and claims of theft. The issue is similar to clothes. The SMART guideline has:

If the clothes worn are fairly standard, e.g., a simple pair of pants, then a sample of the items can be weighed, and that weight subtracted from the weight of each child wearing similar pants. Such items usually weigh less than 30g, which is less than the 100g increments of the scale. Nevertheless, such a weight must be subtracted from each child’s weight before analysis, because the “rounding” to the nearest whole scale division may have been affected by the pants. (page 68/9)

You could do the same for amulets.

I hope this is of some use.

Many thanks for your answer Mark