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Purity of gold is measured in karats which represent how much pure gold is in a particular item. Karats range from 24 karat gold down to 10 karat gold. Any item with less than 10 karats gold content is generally not acceptable as jewelry.

It is supposed that 24 karat gold is far too soft and supple so it is mixed with silver or other harder precious/semi precious metals in order to give the gold a little more durability/hardness and to modify other metallurgical properties or to control melting point or to create exotic colors. Further, during jewelry creation gold items requires soldering (for setting stones, mounting crowns or creating prongs) for which non-gold metals are used reducing purity of gold again. Generally cadmium soldering reduces it to 22 karats while others tend to reduce it to 20 or less. 18 karat has been found to be perfect for diamond settings. In this way gold jewelry available for sale bears 22-18 karat gold content. Today 18 karat jewelry is becoming popular as it is not only stronger than pure gold but looks like gold and is economical as well as durable.

Laboratory tests are perfectly reliable test for determining the purity of gold. Though individuals may like to check the available laboratory reports yet they do not prefer to go for a lab-test everytime they buy/sell it. Due to practical problems and legal complications it is not so popular. Generally lab-tests are opted only when some legal issued are involved.

A quick and non-destructive technique suitable especially for relatively flat and large surfaces is modern X-ray fluorescence technique which has an accuracy of 2-5 parts per thousand and the results can be automatically printed by a computer. It can measure the presence and the content of other alloying metals. It is not suitable for item with chemical surface treatment or electroplating.

Jewelers with a higher reputation puts a Hallmark on the jewelry created by their channels/associates which guarantees the purity of gold content. While others put their own mark followed by a “k”. It is interesting to know that jewelrs mark their gold differently in different countries. In some countries jewelrs mark a piece of jewelry with a number like 18, 20, 22 followed by a “k” while others use their markings which represent the percentage of gold content such as 90%, 87.5% To determine the purity of gold we should first understand this pattern and then find these marks which may appear on the clasp of the piece or some lesser important areas of the jewelry. If we do not find any mark possibilities are that it has been prepared by a local goldsmith or it might be a gold plated item or a unreliable/inferior quality jewelry piece.

After examining the marks as above first of all look at its color. It must be bright yellow which clearly means that more yellow the gold more pure it is. Though by adding different metals and alloys to gold many other colors can be created. White gold is created by combining gold with zinc or nickel while rose colored gold uses copper and green gold uses an alloy of silver, zinc and copper. When these metals are mixed with gold it causes a different hue and their purity can only be determined by color specific methods. In general colored gold seldom contain more than 18k purity.

Decorative enamel enhances the beauty of a piece of jewelry. It costs buyer at par with gold as its weight is included in the total weight of that jewelry item. This decorative enamel has no metallic value and at the time of resale/redesigning/recycling its weight is excluded from the total weight of the jewelry item. So take this factor into account while buying a jewelry item with a decorative enamel.

Widely available/acceptable method of determining the purity of gold is Touch Stone Method. In this method item is gently rubbed against a special stone treated with acids and the resulting color compared to references. Many experts/experienced goldsmiths can predict its purity simply by seeing such rubbings. This test can establish with confidence the smallest difference in precious metal content. It does not require sampling by destructive means such as scraping, cutting or drilling. Testing of very valuable pieces is possible without hesitation and without any compromise with its originality and weight. So it is widely acceptable throughout the world.

Totally destructive yet 100% reliable with an accuracy of 1 part in 10,000 is cupellation wherein the piece consisting of gold and other base metals is placed in a crucible melted on fire and allowed to freeze. When solidifies pure gold is separated from this slag using some simple techniques. Generally only a few pieces from a lot are randomly selected or a little sample piece is taken from the finished item for this test. It is suitable for bullion and gold stocks and not for works of art or jewelry.

Another simple but reliable method of testing purity of gold consists mainly of acids/chemicals. However please note that any testing of gold with acids/chemicals may mean a loss of some small amount of gold.

Under stretchability tests material's ability to be stretched into a wire under tensile stress and/or material's ability to form a thin sheet by hammering or rolling under compressive stress are tested. A single gram of gold can be beaten/hammered/rolled into a sheet of 1 square meter or thin enough to become translucent transmitting greenish blue light. Such semi-transparent sheets also strongly reflect infrared light which makes them suitable as infrared shields in visors of heat-resistant suits and in sun-visors for spacesuits.

Now a days electronic testing is becoming popular as it is non-destructive, simple and accurate.

Some people are of the opinion that 24k gold is not pure gold as it may contain a very little amount of copper in it. Absolutely pure gold is so soft that it can be molded with the hands.