Diablo – What gem qualities are there? What can be found and what has to be crafted

Shortly and simply: no restrictions. :)

Quotes from wiki, who quoted blue posts:

They don’t have a level requirement so we do intend to see them used as a way to twink new characters, or allow people to buy into gemming up a bit earlier on if they have the gold.

There may be some interesting complications with this though.

You can pay an artisan to remove the gems from an item. The last design I had heard of was that it was based on gem value, so as you socket higher level gems it becomes more expensive, but you’ll almost certainly want to unsocket gems to level them up, or swap to new gear. This may cause some unique problems for low level (non-twink) characters attempting to buy high level gems and then being unable to remove them from the socket when they get a better piece of armor (because they can’t afford it), but that may turn out to be an acceptable roadblock.

Source: http://www.diablowiki.net/Gem

There is no character choice that you can make which cannot be easily changed later on, except for the name and gender :)

• Skills and runes are unlocked as you level up; as you have correctly observed, you just select the active ones, not make permanent choices.
• Attribute points are given as you level up, and are consistent and not chosen or randomly allocated.

The only permanent choices you can make are item-related ones - if you salvage an item, for example, it's gone forever.

By the way, because of this, there is practically no reason to create two different characters of the same class if you just want to experiment with a different build - unless you want to experience the game again from the start as a new character, or want to play a different gender, or want to create a hardcore character.