(The story below is not part of the puzzle. If you want to jump straight into the challenge, you can skip ahead to "The Island")
It's been a day just like any other day of the week. You have been working from dawn to dusk and nothing special happened either at work or during the time you spent driving to work and back home. As you arrive at your doorstep you see an envelope laying on your doormat. The only thing written on it is your name.
"Strange... who would leave me something like that?" you think as you pick it up and go inside to open it.
In the envelope you find a letter and picture of your grandparents. The letter reads as follows:
It's been a long time since we've last met. Your grandma and I are missing you a lot. Would you like to visit us this weekend? I've got something great to tell you!
Do you still remember the pirate stories I used to tell you when you were a little boy? And that you've always wondered if there is a little truth in any of the stories? A few days ago I found a map on our attic in a chest that was hidden in a corner and all dusty. Apparently it's a map to the location of Pirate Blurpwhicks Level. It was one of the stories I told you!
It would be a great pleasure if you would accompany me on a treasure hunt!
Grandpa and Grandma
"Wow!" you thought. "Could it be that grandpa wasn't making up all the stories he told me?" And so you decide to pay your grandparents a visit on the weekend.
After spending all Saturday with your grandparents talking about your life and what you have been doing since you've last met, you decide to ask your grandpa about the letter he wrote.
He smiles and is really excited as you both are talking about the treasure hunt. And so you both decide to start early the following morning.
Based on the map that your grandpa found in the chest in his attic you have no problems finding the island, which is located only a few miles away from the coast of the small town your grandparents are living in.
With the map in your hands, you follow a winding path which seems to lead deep into the forest. At the end of the path, in a small clearing, you find a somewhat big mountain, which you had not been able to see from the shore of the island. You are able to discover an entrance into the mountain and end up in a circular room.
On the opposite wall of the entrance is a door. There seems to be something engraved into the stone door, but it only looks like gibberish to both you and your grandpa.
Y G V G R E T N V N Z A F N A N
Since you don't know what to, you start circling the room, looking for some hints on how to open the door. A few steps to the left of the door you find some gems embedded into the wall. All of them seem to be shimmering in beautiful different colors. There are four rows of gems in the wall.
Red Violet Purple Black Blue Green Gold Indigo White Pink Silver Yellow
The gems remind you of buttons of some sort. Curious as you are, you want to press one of them just to see what happens, but your grandpa quickly stops you and points to the opposing wall. You walk over to there and find another note carved into the wall.
You who found my secret cave and entered Pirate Blurpwhicks Level be warned! This room is guarded by my thirteen fallen companions of war! Do not dare to touch a gem if you don't know what you are doing! Pressing only a single wrong gem will make the entrance collapse and bury you inside the cave where you will rot forever or be crushed like a snail shell by the rocks falling down on you!
It looks like you have found yourself in a pretty risky situation. If you decide to press the wrong gem, both of you might not get out of the cave alive. But if you can somehow figure out which gems to press, you might find some great treasure behind the stone door.
Can you figure out what to do and will you be able to get out alive, or even advance through the stone door?
Depending on how well this puzzle is received by the community, I might make this into a multi-part puzzle! I'm hoping for some great feedback, if it was too easy or hard or just right. Also, pardon my english / writing skills as they might not be the best (non native english speaker here :)).