Ever wonder where Garnets visor comes from? Well I think I've worked it out.
Many people think her visor comes from Sapphire. The reason being is that Garnet apears to "summon" it like a gem weapon, so since we know Ruby's weapon is a gauntlet it is safe to assume the visor comes from Sapphire. It's also belived that the reason Shapphire has it is to protect her eye since it is presumably the sorce of her Future vision.
I have a different theory. I belive that the visor is not from Sapphire, but from Ruby, although (like Ruby's gauntlet) Sapphire does have some infuence on it.
Firstly, let's look at Gem visors in general. There are a few Gems that have them, they are Peridot, Doc (Ruby) and Topaz (you could also inclued Jaspers helmet as having a visor if you want). The thing these Gems have in common is that they are all 'military' Gems, Rubies and Topazes are soilders, whilst Peridot (our Peridot at least) appears to be a military technician of sorts.
Now, Peridots we can assume all have visors since they all need eye protection. But what about the soilders. Well I think th visors are used to symbilise rank. That is, Doc has a visor to show she is the leader of the Ruby sqaud.
I think that all Rubies have the potential to summon visors but it is lock away in their Gem code and it gets unlock once a Ruby is promoted. However, our Ruby was never promoted by Homeworld and can't summon her visor on her own. That's where Sapphire comes in. Sapphire is a naturaly high ranking Gem, so it's likely that when Garnet is formed it "tricks" Ruby's Gem into unlocking her visor so Garnet can summon it any time she likes.
But why is it opaque?
Well, thats the "Sapphire influence" I mentioned. Whilst I don't beleve that the visor comes from Sapphire as eye protection, I do think there is some truth to it.
Sapphire almost always has her eye covered, so whilst I don't think this is to protect from harm, I do think that it is important to keep her eye covered for her future vision to work. I think that Garnets visor serves a similar funtion. Keeps her eye covered so she can use her Future vision.
Sugilite dosn't have her "Sapphire eye" covered, but then again she doesn't seen patient enough for seer powers.
And Sardonyx? Well that's a whole other theory.