Silver
There are several kinds of silver.
Fine silver is 99.9% silver or better. It is too soft for general use. Mexican jewelry marked Sterling is made according to the Sterling standard (92.5% silver).
 

Coin silver is most commonly 90% silver and 10% copper as dictated by United States FTC guidelines. The name comes from the practice of melting down silver coins to make objects. Coin silver is usually lower than sterling silver.
 

German silver (from Germany) is marked 80 or 900 indicating 80% or 90% pure silver.
 

Sterling silver is 92.5% silver and 7.5% copper.

Argentium Sterling Silver is a modern sterling silver alloy which modifies the traditional sterling silver alloy (92.5% silver + 7.5% copper) by replacing some of the copper with a rare metallic element called germanium. Argentium sterling silver has high tarnish resistance.

Adding additional elements to the silver give it hardness and durability. Problems with corrosion and tarnishing increase as the purity of the silver decreases. Chemically, silver does not react with oxygen or water at ordinary temperatures. The other metal in the alloy, usually copper, may react with oxygen in the air to form tarnish. Common table salt is known to corrode silver-copper alloy. Harsh polishing and buffing can permanently damage silver and damage stones in jewelry. An easy way to shine silver is to take some dry baking soda and rub it on the metal with one's fingers, then to rinse it with mineral-free water. Polishing cloths as© "Sunshine Cloth" can be used to polish silver jewelry and avoid rubbing and injuring the stones.

Gold
Since pure gold is too soft for practical daily wear; it is usually mixed with silver, copper, and other metals to make it more durable (and to lower its cost). Pure gold doesn't react with other elements to create tarnish, the residue that accumulates on some metals before transferring to your skin as a stain. Sometimes people have an allergy or staining problems with metals that are combined with gold. Gold can be worked into nearly any shape, including tiny strands that do not break easily.

Jewelry is marked 18K, 14K, or 10K, with the K standing for karat, the system used to describe the percentage of pure gold an item contains. The higher the karat number, the higher the percentage of gold in gold jewelry. 24K gold is pure gold. 18K gold contains 18 parts gold and 6 parts of one or more additional metals, making it 75% gold. 14K gold contains 14 parts gold and 10 parts of one or more additional metals, making it 58.3% gold. 12K gold contains 12 parts gold and 12 parts of one or more additional metals, making it 50% gold. 10K gold contains 10 parts gold and 14 parts of one or more additional metals, making it 41.7% gold. 10K gold is the minimum karat that can be called "gold" in the United States.
 

European gold jewelry is marked with numbers that indicate their percentage of gold, such as: 18K gold is marked 750 to indicate 75% gold 14K gold is marked 585 for 58.5% 12-karat is 50% gold and 10K gold is marked 417 for 41.7% Karat marking on gold jewelry should be accompanied by a hallmark or trademark that identifies its maker and/or the country of origin..
 

Gold filled or gold overlay is made by heat and pressure bonding a thin layer of 14K or 12K gold to a brass core. 1/20 refers to the ratio of karat gold to brass core by weight, which is 5%. Newer gold filled items have markings that indicate how much and what type of gold was used for the layer. A marking that says 1/20 12K G.F. means that the jewelry is at least 1/20th 12K gold by weight.
Gold Plated Jewelry The gold layer in gold plated jewelry is thinner than the gold in gold filled jewelry, so it usually wears away more quickly. Gold washed is used to describe a very thin layer of gold which won't be very durable.

Titanium
Titanium is extremely elastic (springy), lightweight, corrosion resistant and non-magnetic resulting in use for jewelry (titanium rings and earrings). Titanium is used for jewellery because it is regarded as hypoallergenic and does not discolour skin. Pure titanium dioxide is relatively clear and has an extremely high index of refraction with an optical dispersion higher than diamond. Titanium dioxide is produced artificially for use as a gemstone, but it is relatively soft. Star sapphires and rubies exhibit their asterism as a result of the presence of TiO2. Titanium can be alloyed with other elements such as iron, aluminium, vanadium, molybdenum and others, to produce strong lightweight alloys for jewelry.

The two most useful properties of the metal form are corrosion resistance, and the highest strength-to-weight ratio of any metal. In its unalloyed condition, titanium is as strong as some steels, but 45% lighter. Titanium is used in watchmaking for the production of watch cases. Watchmakers appreciate titanium for its durability, light weight, dent- and corrosion- resistance. Titanium watches are often coated with a protective material to make the surface more scratch-resistant.
(Information gained by researching the internet.)

Gemstones
Semi-precious gemstone is a misnomer, but it commonly refers to all gemstones other than ruby, emerald or sapphire.
Information about gemstones can be found at International Colored Gemstone Association

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