Recognizing a good avocado
Let’s start by specifying that we can find two types of this wonderful fruit: a type with a more rounded shape and a rather smooth skin and an intense green color; another type with a slightly elongated shape with a rougher skin and a darker color, tending to black. This, however, does not indicate the ripeness of the fruit but simply indicates two different types of avocado.
Both are very tasty even if the taste is a little different.
What you should rather pay attention to is:
The avocado must have the stalk: this, in fact, guarantees the good ripening of the fruit as it prevents the entry of air. An avocado without a stalk may be slightly black on the inside or not ripened evenly. If the dot under the stalk is still yellowish-green the avocado is perfect to eat, but if it is brown the fruit is probably overripe or rotten A ripe avocado has a stalk that comes off easily Do not use a knife or other tools to cut the petiole. If you can’t remove it with your fingers, the avocado is not ripe and not ready to eat.
Another important factor is the consistency: the avocado must not be soft, too soft, but it must still give slightly to the pressure of your fingers, like a ripe peach so to speak. Gently shake the avocado. Bring it close to your ear and shake it slowly a few times to hear if something moves inside. The inner bowl detaches from the pulp when the fruit is overripe. As a result, the fruit makes a noise when you shake it.

What to do if the avocado it's not ripe
If you have cut the avocado but it is not ripe yet, don’t worry you can rhyme are: sprinkle the inner surface of the avocado (including the stone) with lemon or lime juice, close the fruit and cover it with cling film. put it in the fridge and after a few days, it will be good to eat (unless it was too far from ripening).
How to make it mature faster
If you want an avocado to ripen faster, put it in a paper bag with two apples; ethylene, a gas released by apples will make it ripen earlier.