The smugglers tattoos

Describe a common item that is used as a secret symbol in your world. Tattoos.
Tattoos in general are a fairly common form of art in the kingdoms of the Ukari and can range in complexity and design. Many of the tattoos have symbolism to the owner as an animal that represents their house.
The special Tattoos: The tattoos will be used to identify members of the Smuggling group who travel into the northern reaches in search of forbidden treasures and items.
What is the ink used in the tattoos and where does it come from? A vibrant mushroom from the northern reaches of Zurrican is used in making a tattoo that will change colors when exposed to a certain light from the crystal shards of the swamps. As the tattoos ink is rare to make and find the materials it is used to mark the Stealthy smuggler group who sneaks into the dangerous northern territory in search of exotic and forbidden treasure, plants and animals. The special ink is tattooed into a specific tattoo of their choosing that already exists. The ink does not change the color to the naked eye so only when a Swamp Crystals glowing light is exposed to the ink will it show giving off that this individual is a member of the smuggling group.
The swamp crystals are banned and very rare so the tattoos can not be checked by most of the population. One such swamp crystal does exist with the Ukari Lawbringer who is tasked with checking possible smugglers with the crystal to see if they have been marked as one of the group.
Being caught as a smuggler has serious ramifications and consequences.
Basically one of two options may be chosen as punishment.
1] the tattoo is burned as to remove the flowing effects of the ink and to symbolize the individual as giving up the group of smugglers.
2] if they choose to decline they will be tattoos in a bright red ink across the front of their forehead. this tattoo will show the rest of the nation that you are in fact a criminal and someone to be cautious around. effectively making it impossible for you to hide anymore.