She said the box couldn't hold her but before it was stated that it was about a person's size.
A jewel cannot have as much worth as a person ... well, if there is slavery in that world, she probably will not amount to much compared with a blue diamond the size of a person. This also gives us another piece of information: if there is diamonds so huge, a normal sized diamond should be quite cheap so even middle class people could have one ...
Also, it is surprising that we were shown already Peacock's item in chapter 30. Something for the novel readers, maybe?