the resin. They know you have to break the resin to get it tested and people will not do that, because it will destroy it's re-sell value. In my case
the necklace broke, so I could send it for testing. This is a very old trick used to fool tourists visiting foreign countries and also people buying things
online.
Example : https://exportcity.trustpass.alibaba.com/product/109094522-230781803/Real_Butterfly_Jewelry_TRIM_plated_24K_gold.html
The ad says its gilded/gold plated, not made of gold. Gold plate isn't typically worth anything for gold value anyway, its purely ornamental and worth what you think its worth for appearance.