I believe that this item should be tagged as "Consumable."

Sure you don't consume it as in eat it, but the item is consumed when you activate it.

i don't think you can control the elemental from the gem

two reasons:

1. according to the DMG rules about creating a magic item, an uncommon item can cast a no higher than 3rd level in a form of one per day and conjure elemental is a 5th level. so it needs some disadvantages to be balanced.

2. the text says you summon an elemental as if summoned by the conjure elemental spell so if you don't really cast the spell you are not concentrating on it and if the concentration is broken it becomes hostile according to the spell.  

1. The disadvantage is that the item can only be used once and you don't get a choice on what is summoned. Also, the DMG's segment on creating magic items isn't meant to be super strict, it's more meant to be a suggested way of how to balance a home-brewed magic item.

2. "Summoned as if you had cast" not "as if summoned by," a small but vital difference. You're correct that you don't technically cast the spell, but the wording implies that you apply ALL effects of the spell, including concentration.

i'd say its the DM's choice if the elemental can be controlled

why is the blue one wind and the green one water? this vexes me it should be switched.

Quote from Walpurgisplay >>

why is the blue one wind and the green one water? this vexes me it should be switched.

 

 If anything, green should be earth. Blue should be water obviously. Yellow being air is perfectly fine.

Just a guess, can't actually pin it down for sure but I've read some silly crystal healing boards and they say that sapphire is often associated with celestial bodies, the heavens and the sky. Also diamonds are well known for being extremely hard but also very brittle and emeralds are often associated with change, growth and rebirth, something that water is also associated with. It's a possible explanation for an otherwise inexplicable situation.

I usually see these used as power sources for larger magic items and other pieces of “GM magic.”

A red corundum is called a ruby. A sapphire is any other corundum.

It says "As if you cast the Conjure Elemental Spell", but does it require concentration?

I guess it does?

Quote from BirdMonk >>

It says "As if you cast the Conjure Elemental Spell", but does it require concentration?

 

I guess it does?

 I would rule that it does.

Alternatively you could change the rarity. 

The gem isn't consumed, it is broken.
After the elemental has been conjured you're left with shattered pieces of whatever gem you had. Use the mending spell and you have yourself a solid gem again, which could be reused for a number of things, or just resold as a gemstone.

Mending "repairs a single break or tear in an object you touch".  It would take some time repairing a shattered object.  Many wizards are bookworms used to spending tedious hours on lower magic, I suppose it suits them.  This assume "breaking" the gem means "shattering" the gem.  I just gave out a fire elemental gem to the party slaying Cryovain (my dragon had treasure, by golly!).  It will be interesting to see how they use it.

The art was clearly made assuming blue to be water and green to be air as well...

I am wondering what type classes would be able to create elemental gems?

DM classes ^^, clearly the magic item balancing is not done. Too time consuming for writers i guess .....

Anyone else feel like the emerald should be air and sapphire should be water?

Edit: Looking closely the gems even have the elements modelled on them. The blue has waves, the yellow has earth spikes and the green has gas

Quote from LegendOfZub >>

The art was clearly made assuming blue to be water and green to be air as well...

 Heh, you're right. The art is inaccurate! That usually only happens on monsters, though....