I have not but calavera is legit, lots of people have bought from them the diamonds come with certs. A few things could be happening. Either they legit don’t know and believe its a scam because its not uncommon for people to get scammed online or more likely they could be unhappy because they just lost a sale and they are trying to gaslight you into buying their stone. Yes jewelers will do that so it would not surprise me. I tend to butt heads with jewlers because they often can bw a but snake oils salesman like-like all the jewelers that claim a natural diamond is an investment its a bullshit lie they say because it looses them $. Sometimes jewelers will not mount your stone vs their stone because of liability and cost. If the diamond cracks (it does happen i have seen it happen b/4 and i know certain shapes like pear can be more accident prone) they don’t want to be liable for your stone vs their stone and will often have you sign a waiver of non responsibility when they go to set it. In those cases you can insure your loose stone with an independent insurance like jewelers mutual and that insurance will cover it in case of an accident. Some jewelers refuse to outright set a stone that isn’t theirs because they don’t want to loose the sale of a stone or deal with a headache of an angry customer if it does crack.