The Crystal Gallery (aka the D-Tunnel) is a side tunnel that branches off of Level U-9. The tunnel consists of fourteen levels, making it the highest number of levels in a tunnel designated with a letter; the tunnel can be played with any character. All the levels were designed by mathwiz100. 1, 2, 3, 10, 11 and 12 come from Topaz, 4 and 5 come from Diamond, 6, 7, 8 come from Emerald, 9 came from Sapphire, and the last 2 levels came from Garnet.
This tunnel is just another one, nothing too special, except the fact that it has many bright designs, patterns, and colors.
The following is a brief description of the story in the Crystal Gallery. More information on the story can be found here.
After beating the D-Tunnel, the cutscene 'Naming' will begin. The Gentleman meets up with the Runner in D-12. He complains about the way she is naming the tunnels, saying that her labels are "arbitrary" and "uninformative", and that they will reflect poorly upon the group when they return to the Planet. The Runner mumbles that it doesn't matter that much, but she allows him to name a tunnel or two. Then she runs the other way while the Gentleman considers the different factors needed to make a good name. Afterwards, the screen cuts to the map, where the player can see that the D-Tunnels have been renamed "The Crystal Gallery" by the Gentleman. Two text bubbles will pop up on the bottom of the screen showing a mental note by the Runner. The text bubbles say "Note to self..." and "Don't let the Gentleman name any more tunnels," respectively.