Heavy silver bangle with unknown stone-- unique markings on inside and out.

Hi. I went into a jewelry appraiser and wasn't planning on buying anything but as soon as I saw this bangle I had to have it. This is all I know. It's super heavy. About 125 g of silver. The stone changes from that bright blue to dark in different lights. It has super weird carvings. Snakes? Octopus? Lots of detail work. My questions are--- is this vintage? I mean it sure looks it. Does anyone know what stone this is? And he was asking 550 for i got him down to 275. I am not into any of this stuff so don't know if this a rare or cool find to you guys, but I love it! Any help would be greatly appreciated. Purchased in Clearwater Florida.