I don't have a ton of experience with gem colors, but from an artistic perspective I'd probably say Royal Blue or Indathrone Blue (but that's pretty obscure!) To me, navy is much darker and blacker with less blue pigment, while cobalt would be lighter and more vivid.
I would try to stick with names to describe the saturation and tone, like Freke did. I feel terms like navy, cobalt, royal blue, etc are not helpful. It's like saying a diamond is lemon, when you really mean fancy intense yellow. The latter means something to other people familar with gemstones, and helps to determine a value.
Pregcurious, you made a really good point about using actual terminology! I was assuming he was trying to describe it to a layperson. In my head, he was excitedly trying to describe it to a family member or friend, and was looking for something descriptive and evocative. But I probably read WAY too far into it
On that note, I double checked my paints, and misspelled the color in my first post- it's actually indanthrene!
You are right, I was trying to describe it to a lay-person (I am female by the way!).
However, I like the idea of being more specific. Do you all agree this describes my sapphire best (taking into account computer color differences of course): Blue hue dark-very dark medium to strong saturation natural corundum?