How To Tell If Gold Is Real
Of all the subjects one can possibly research online, the subject of how to tell if gold is real must rank as one of those highest in either misinformation or partial truth. To illustrate the misinformation that is out there - I have even read one statement that if gold jewelry sinks in a bucket of water, it must be real! This is obviously false - almost all metals are heavier than water.
There are many tests for gold and quite a few of them eliminate some but not all fakes: What is needed is a comprehensive resource and, depending on the circumstances, more than one test. So with this page, we have attempted to build the "mother ship" - a giant list of all the possible ways to tell if gold is real, together with descriptions of their limitations.
Important note - if you are examining gold of considerable value, the best way of all to ascertain whether it is real is to take it to a reputable professional jeweler or antiques dealer, or to contact an assay office (most countries have these). A good jeweler / antiques expert will have the training and the "lookup charts" to decode accurately hallmarks on jewelry. Also a reputable jeweler will have testing kits to evaluate gold and of course the expertise to test it without causing damage. A jeweler may charge a small fee for this service.
An assay office is an institution specifically set up to test the purity of precious metals. It is only assay offices that are legally permitted to apply hallmarks to gold, platinum or silver items, guaranteeing their purity.
As assay offices may be considered the greatest masters of evaluating gold, it is instructive to observe their methods, as these will be the ones of greatest accuracy. We will also look at simpler methods which may be applied at home, with consideration of their accuracy and usefulness.
Tests For Gold
The first observation is that some methods of evaluating gold are more destructive than others: If one has a piece of fine jewelry, one wants to use the least destructive method possible, whereas if one has raw metal, this factor is not quite as important.
A few keen observations will often give some good initial hints as to the genuineness of gold:
First, gold bullion bars will very often (but not always) come with an assay certificate guaranteeing their purity. It is also recommended to buy gold bullion of one of the well known brands (typically major banks or well known mints), through an established bullion dealer.
A genuine, solid gold nugget: Note the 'rounded'
appearance of pure raw gold: No crystalline edges.
1) Feel the weight. Although not an absolute test, solid gold is heavy. (It has a specific gravity of around 19.2). A gold plated copper, iron, brass or silver item will simply not have the same weighty feel to it. However lead and tungsten are both heavy metals.
2) Examine any hallmarks and other marks closely, with a magnifying glass or loupe if possible. A genuine hallmark will have a professional appearance - evenly stamped and detailed. Check against a hallmark chart (which can be found online). A loupe can be obtained for under $3 (w/o shipping) on Amazon.
3) For gold nuggets: Check the hardness. Pyrite will scratch glass or a penny, whereas gold will not. (Pyrite has a hardness of around 6.0 to 6.5 (Mohs), Gold 2.5 to 3.0). In old times, people used to bite gold to tell it from pyrite or other metals, as gold is soft enough to be marked by teeth. However this is not recommended for four reasons a) you could damage your teeth on a hard substance b) other metals (such as lead) are also soft c) you might be biting something that has toxicity (lead again) or is unhygenic d) A person may not appreciate you biting into their item of gold!
4) Pure gold nuggets have a certain appearance. Examining pyrite with a loupe will reveal a surface made of tiny cubes or similar polyhedral structures. Gold nuggets will generally have a rounded, cornerless appearance, (see image) - if it has sharp crystalline facets it is probably pyrite - although it is possible that a nugget may contain both gold and pyrite. Pyrite will also shatter under a hammer, whereas gold is very malleable and will bend or flatten.
A touchstone is a small, flat block of dark colored stone - for example slate. The stone has a finely grained surface, upon which true gold will leave a visible golden mark if rubbed against it. If the mark left is black, the sample is not gold but iron pyrite - "fool's gold".
When different chemicals, such as nitric acid or aqua regia are applied to the mark left by the gold, they will react differently depending on the purity of the gold. 
This test can be performed at home by purchasing and using a surprisingly inexpensive kit (for example this one on Amazon which costs only $14.25!) These kits contain small bottles of chemicals and a touchstone and will identify 10k, 14k, 18k, 22k gold, plus also platinum and silver. The test works by dropping a drop of the acid on the streak plate on the place where the gold has been rubbed. If the mark stays the same color, the gold is pure. If it turns yellow-orange, the gold may be less pure. Any other color denotes another metal.
The touchstone test has been used for thousands of years including, astonishingly, by the Indus Valley Civilisation in 3,500BC!  The modern day touchstone test kits can identify different purities of gold - however it is not infallible: If the item has a thick layer of gold plate and the metal underneath is not revealed, the touchstone test will only reveal the composition of the plated surface. The touchstone should work well for raw gold nuggets.
Nitric Acid Test
Note - this can damage the appearance of jewelry; use either on an inconspicuous area or on scrap gold only. To perform this test, make a scratch on the metal item using a nail file. Using a dropper, apply a drop of nitric acid to the metal. No reaction - gold (or false nitric acid!) Milky color - gold plated sterling silver. Green color - gold plated base metal. Important safety note - nitric acid is a hazardous substance - highly corrosive, possibly fuming and can also be explosive under certain conditions. It can also be self-igniting when brought into contact with certain chemicals. It should only be handled by someone who has the requisite experience and safety equipment.
Streak Plate Test
An ultra-simple version of a touchstone test, which may be performed at home to evaluate gold nuggets, uses what is known as a "streak plate". This is simply an unglazed piece of white porcelain tile, perhaps a tile's back surface that is not shiny. Gold will give a golden streak on the plate, pyrite a black streak.
X-Ray Fluorescence Test
This is one of the tests that may be performed by assay laboratories. It requires expensive, hi-tech equipment. The material is exposed to short wave X-rays and a computer spectrum reading will indicate the presence of not only gold but also other metals. This test is not recommended however for plated or surface treated items, as it will give a reading of the metal at the surface of the item.  
Pure gold has a known density, therefore if the volume of the gold object can be calculated accurately, (perhaps by submersion in water, although the water displacement would have to be calculated with extreme accuracy) the weight can be extrapolated from the volume and compared to the actual weight of the item. This test was allegedly used by the famous Greek scientist Archimedes, who was asked whether he could tell whether the King's crown was pure gold without damaging the crown. However, a more elaborate version of this test, which uses a buoyancy principle now known as Archimedes Principle, is more likely to have been used as it is more accurate and more simple to perform. 
Cupellation / "Fire Assay"
This is the most accurate of all the tests for gold. It is however destructive of the sample being used. It is the method used internationally in the gold mining and refining companies and can ascertain the purity of gold ore. It is used to assay gold bullion and when properly performed can have an accuracy up to 1 part in 10,000.
This is a basic summary of the fire assay process: The sample is wrapped in a lead foil together with copper and silver. The sample is heated at 1650°F in a cupel (a porous pot) made of bone ash or magnesium oxide. Base metals will oxidise and absorb into the cupel. The product is then treated with nitric acid to remove silver. Precision weighing of all the components will ascertain the amount gold, silver and base metal in the sample. However, if there is much base metal or ore in the sample, additional steps may be required as the cupellation can only absorb a certain amount of base substance from the sample.  
The method used for fire assay is old in essence, and in modern times differs only in detail from methods delineated by Agricola in his famous 1556 metallurgical treatise De Re Metallica.
How To Tell Gold Plate from Solid Gold
If gold plating is of high quality it can be difficult to tell from solid gold. Here are a few additional checks that can be performed:
1) Look at the sale price of the item. If it is less than the current price-per-ounce of gold, it is very likely not solid gold (why would anyone sell real gold at less than the spot price when they can immediately sell it for a price very close to the spot price to a scrap gold buyer?)
100 Mills Gold Bar
This bar is gold plated only!
Designed to look like a genuine
gold bullion bar, even to the point
of being seen cased in plastic
- and also weighing one troy ounce.
2) Look for marks identifying the gold as gold plate. If you see the markings RGP ("Rolled Gold Plate"), GE, HGE, GF, or 1/10, 1/20, or 1/30 then it is gold plate. Also if you see anything with "100 Mils", "100 Mills" ot "5 Microns" on it, on either the item or the sales description, this is a description of the plating and means that the item is plated or clad to that depth. The core is either silver, brass or some other cheap metal. Similarly, if you see the words "Gold Clad", "Layered Gold" or (obviously) "Gold Plated" in the description, then the item is not solid gold. Beware however of the mark "KP" - for example 14KP - it does not mean "14 Karat plated", but means "14 Karat Plumb", which means "14 Karat exactly, no more, no less - the item is (claimed to be) solid gold.
On the other hand, hallmarks and / or millesimal fineness marks (a three-figure number such as 750, 585, 833 or 917) are marks meaning that the item is indicated to be gold. However if the item is marked "sterling" or 925, it is sterling silver that has been gold plated. Not all gold items will have hallmarks.
3) Look for areas where the gold plate may have worn off, such as prominent edges or clasps. If the jewelry is old and has seen much use, it will have had some wear and gold, being a soft metal, can wear down over time simply through friction. If another colored metal shows through in a prominent place, the item is plated. Green or black corroded spots will also indicate the presence of other metals underneath.
4) Run a magnet over the gold. If the item contains iron or nickel, it will be attracted to the magnet, whereas pure gold and many other metals will not. So while this test may be useful in sifting out some fakes, "just because it passes the magnet test, doesn't mean it is pure gold".
How to Tell If Gold Is Real - References:
 Bisht, R. S. (1982). "Excavations at Banawali: 1974-77". In Possehl, Gregory L. (ed.). Harappan Civilization: A Contemporary Perspective. New Delhi: Oxford and IBH Publishing Co.. pp. 113–124.
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