Diamonds may be the hardest substance known to man and one of the most durable but that doesn't mean they are unbreakable or completely scratch-proof. You can, for example, chip a diamond if it makes contact with a hard surface with sufficient force. Such an occurrence would certainly significantly reduce the value of the stone.
A key component of diamond protection is the setting it is in. A diamond that is mounted on, say, a ring that has elongated prongs is, as you can readily imagine, at more risk of being damaged or being popped out of the setting than one with a more modest setting. Keep that in mind when deciding on what you would like your ring to look like.
It is not widely-known outside of the jewelry industry but diamonds can be discolored, particularly by chemicals like chlorine or those found in hairspray. Brief contact with those compounds is not usually a problem but a gradual accumulation of these chemicals could discolor the gem.