Anna is absolutely right on this – it is dyed beryl. This is the way it goes – they (manufacturers of these items) take low-quality beryl (usually white) and dye it with green color. When it is dyed, it is impossible to tell what the initial color was. Now it is green and it is beryl. As you may know, green beryl is called emerald. However, not when it is dyed. They can call it whatever they want, but it is nothing more than cheap, dyed beryl after all.
What amazes me is that Catawiki’s “experts” keep on accepting and listing these items for years. And they allow labeling these items as emerald necklaces, bracelets, etc. Could it be that they don’t know what it is? Possibly, but then they are no experts. Could it be that they do know what it is? Possibly, but then they are fraudsters.
This listing made me laugh. Through the tiers, though. Catawiki auction site proudly presents itself as listing only unique and special objects. They say they have professional experts vetting every submitted item. They say all the auctions are supervised by a notary public. And they do not allow to use stock images in the listings. What a shame! What a shame when you compare their words and actions! Here is the original listing of this item on eBay (with the same images :)): https://www.ebay.com/itm/Green-Emerald-1161-50-Cts-Earth-Mined-4-Strand-Round-Shape-Beads-Necklace-Rare/362439204534? (just in case this listing gets removed by eBay, I attach the screenshot here, too).
So, to cut the long story short, I am happy for you, Martin65, that you didn’t bid on this item. I have no intention to say that everything is fake and bad on Catawiki, but you have to exercise due care because most of their “experts” are good for nothing. Or they are instructed to close their eyes to many things. Who knows…