For judging the authenticity of a string of hawksbill bracelets, it is impossible to accurately judge only by the size of 20 mm, because there are many criteria for judging the authenticity of genuine hawksbill bracelets, such as material, color, texture and so on. The following are some common methods to judge the authenticity of tortoise shell bracelets:

1. Material comparison: genuine hawksbill bracelets are generally made of genuine hawksbill bracelets, with hard materials and special lines. You can compare it with the real tortoise shell to see if the color, texture and texture are similar.

2. Price reference: Tortoise shell is a precious material, and the price is relatively high. If the price is too low, it may be a fake tortoise shell or a bracelet made of other materials. Need to be wary of low prices.

3. Purchase channel: Choosing a formal and credible merchant or brand to buy tortoise-shell bracelets can provide formal shopping guarantee and quality assurance. Buying bracelets in some informal channels may be risky and it is easy to buy counterfeit products.

4. Professional appraisal: If you have doubts about the authenticity of the bracelet, you can consider getting a qualified jewelry appraisal agency for professional appraisal, and they will conduct the authenticity appraisal through professional instruments and experience.

To sum up, it is impossible to accurately judge the authenticity of the bracelet only by the size of 20 mm It is suggested that you make a comprehensive judgment according to the above methods, or seek the help of a professional appraisal agency to ensure that you can buy a real tortoise shell bracelet.