I've been seeing a lot of Peruvian Opal for sale online, it comes in
all kinds of blue, green or in-between colors. It reminds one of Gem
Silica, and one site stated that both of these stones are mined in
the same areas of Peru.
There is a sea-green dyed chalcedony out there also.
Opal is less dense than chalcedony, maybe you could make an
identification with the help of a scale. If it were Gem Silica, you
could tell by the price!