With that you can make plain crystals or bricks.
Which, at this point, aren't worth much beyond projects... but when a project needs them they are always in high demand.
You can use it with 7 barnacle talc in the crystalmalizing chamber to make a plain crystal.
You can use them to make bricks if you have the block making skill...
You can sell them on the auction or to a vendor (but you will get more currants on the auctions).
You can set them in your house as a decorative item...
Yep, open the jar...they will fly out...I don't know what happens if you try to open the jar and you aren't near a firefly swarm though (for example, opening a jar in an Alakol home...will they stay there, or disappear, or not leave the jar?).
just bought 5 firefly jars. all 6 will look good in my house.... now could they make the backround look like the game time? so at night it will be more cool.