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.
An ammonia solution – one part ammonia to six parts water – is your best bet for cleaning diamonds. Apply it to your diamond jewelry and gently massage it by hand to loosen any dirt or contaminants. Do not use any commercial cleaning solutions or compounds that are not made specifically for jewelry – even if they say that they are “soft.” They will not restore the sparkle the way that an ammonia solution or a professional cleaning done at your friendly, neighborhood jewelry store can. Be careful, though about applying this solution to other gemstones as they have other properties and not all react well to ammonia. Consult with a professional if you have questions about how to treat other gems.
It is also important to store your jewelry properly. There's a saying that only another diamond can break a diamond and it is somewhat true. (Other things can certainly break a diamond.) But you don't want to take a chance with yours so don't simply toss it into a pouch or box or drawer with other jewelry. Wrap the item in a soft cloth, like the kind often provided by jewelry stores. At the very least you could put each piece of jewelry in a small plastic bag so that it will have at least a minimal amount of protection from collisions with other jewelry. Remember, you have invested quite a bit of money into that diamond jewelry. Treat it properly and it will last for many generations.