Custom machined aluminum metal and glass vial with removable metal plug top and faux leather necklace cord. Plug is removable and creates a seal. You may order the vial with or without liquid. I'll add a green UV reactive dye to the liquid vial option.
If you have a yellow/green highlighter you may try to create the liquid yourself by letting it dissolve in water. There are also more expensive options on amazon of dyes that are concentrated uv reactive liquid. I'll be using those to make a batch of pre-filled liquid vials which you can order here for the additional cost. The one shown in the photo now just shows the green/yellow food coloring dye I used but I'll be shipping uv reactive filled ones if you order 'add liquid'.
About 3/4" x 2" overall.