Lightly grease a 9x13" heat-proof baking dish with neutral-flavored cooking oil or spray. Set aside.
Add 1 3/4 cups cold water to a small or medium saucepan. Sprinkle 1 1/2 Tablespoons agar agar powder over the water, and let sit for 5 minutes. Heat the water over high heat to bring to a simmer. Reduce heat to medium-low and simmer for 3 minutes, stirring constantly with a spatula.
Slowly pour in 3 cups granulated sugar, and continue to stir and simmer for 3 more minutes, until all of the sugar is completely dissolved. Remove the pot from heat and stir in 1/4 teaspoon candy flavoring and 1/4 teaspoon citric acid powder.
Pour the agar mixture into the prepared baking dish. Using toothpicks or skewers, swirl one or more gel colors into the mixture until you are happy with the appearance.
Being careful not to agitate the dish too much, move the baking pan to the fridge, and chill uncovered until it is fully set (1-2 hours).
Line 2 sheet pans with parchment paper and set aside. Remove the baking dish from the fridge, and carefully loosen the edges of the jelly with a spatula. Turn the jelly onto a cutting board (don't worry if it breaks).
Carefully break or cut apart the jelly into 1.5-2" (~3.5-5cm) pieces. I prefer to break them apart with my fingers, as it creates more natural-looking rock-like shapes. If you choose to cut, cut the pieces into irregular shapes. Place the pieces on the prepared baking sheets. If you like, lightly brush edible glitter onto the edges of some or all of the pieces for a little sparkle (note that some edible glitter has a "safe limit" and should be used in small quantities).
The crystal candy can be eaten immediately, but letting it dry, uncovered at room temperature causes it to get a crystal-like crust on the outside. The longer you let it dry, the crunchier it will become. After it's as crunchy as you want it, store any leftovers in an air-tight container or zipper bag.