Your jewelry should be checked at least twice per year, three to four times per year is even better.  I'll do it for free and it only takes a few minutes.  Stones come loose and get damaged, prongs wear out and bend.  Catching these problems in time, before stones fall out and get lost, will save you money and heartbreak.

 Diamonds 
   Diamonds rarely get scratched but do occasionally get chipped.  Diamonds get scratched when they get hit by other diamonds, this usually happens when a piece of diamond jewelry is thrown into a jewelry box with other diamonds in it.  Diamonds get chipped when the receive a sharp blow.  Diamond cuts with sharp points like marquise or princess cuts usually get chipped on the points.  The girdle (edge) is where most other chips occur.  Scratched or chipped diamonds can be repolished.
   When cleaning a diamond a small brush works well, an old tooth brush is perfect.  Scrub down your ring in warm dishwater.  Diamonds are very stable against heat and chemicals.  Grease sticks to diamonds very well, this is what most "dirt" on a diamond consists of.  In diamond mines grease is used to separate diamond from the host rock, the diamond sticks to the grease, the other rocks don't.  This is why dishsoap works to clean the diamond, it dissoves the grease.

Gold
   Gold is easily polished or repaired.  Karat gold will tarnish, chemicals like bromine and chlorine can tarnish gold a dark brown color that can be removed by polishing.  A dip in the pool or hot tub usually is not enough to cause tarnish but repeated, prolonged use can effect jewelry.  If you are in treated water often remove your jewelry before getting in. 
   Gold will wear thin over time, clasps and bails wear through on necklaces, shanks and prongs wear thin on rings and bracelets, and earring backs become loose.  Most problems with wear on gold jewelry can be fixed, have your jewelry checked at least twice a year.

Platinum
   Platinum is heavier and softer than gold, though it will wear out much slower than gold.  Platinum in jewelry is 90-95% pure and will not tarnish, however it scratches very easily, this gives platinum a dull grey look with wear.  Bring in your platinum and I can polish it for free.  Platinum also bends easily so prongs need to be checked regularly.

Silver
   Silver is the whitest of metals used in jewelry.  Sterling silver is alloyed with copper and tarnishes very quickly.  In some jewelry this isn't a problem.  The tarnish will settle into folds and crevasses in the piece, while the higher points remain tarnish free from wear.  Often called patina, this enhances the look of some pieces.  If you don't like tarnish on your silver it is easily removed by polishing or using a product like Tarn-x or Silvo.
   Sterling pieces are often built heavier and thicker than similar gold pieces, this is because silver is softer and less durable than gold.  Remember this when looking at things like sterling chains and go a little heavier than you would with gold.

Colored Gemstones
   There are too many colored stones to list individually here.  If you have questions about a specific stone call me for advice on durabilty and precautions you should take to avoid damaging the stone.
   Soft stones like pearl, turquoise, amber and, lapis lazuli shouldn't be placed in a chemical cleaner or ultrasonic, clean by hand using warm water and a soft cloth or brush.  Sapphires, garnet, quartz, and many of the harder stones commonly used in jewelry are generally stable enough to put in chemical jewelry cleaner.  When in doubt bring it in and I'll clean it for you.