I was calculating the other day and it came to:
1% Armor is about 68.57
To find % Value of your defensive gem. Divide your Gem Defense Value by 68.57
So if we input Supreme Onyx (365 Melee Defense/68.57) it comes to approximately 5.32% so yeah about 5%
PS: This was with a level 49 Character, I'm not sure if its the same for lower levels.