When looking for gold, there are several key characteristics and properties to look for that can help you identify whether a rock or mineral contains the precious metal. In this article, we’ll go over the main signs that may indicate the presence of gold and the testing methods used for detection and extraction.
Color
One of the most obvious visual cues that may reveal the presence of gold is color. Pure gold is bright golden yellow in color. While other minerals like pyrite can have a similar yellowish hue, gold’s color is more vibrant. The hue of natural gold can range from a rich light yellow to an orange-yellow depending on its copper content. The more copper in the gold, the more orange-red its color will be. Natural gold also catches the light and has an unmistakable brilliance.
Streak
A mineral’s streak refers to the color of its powder. To check streak, scrape the mineral across a piece of unglazed porcelain known as a streak plate. Gold has a golden yellow streak which is largely the same color as the natural mineral. By contrast, pyrite has a greenish-black streak. Checking a mineral’s streak can therefore help distinguish gold from lookalike minerals.
Luster
Gold has a metallic luster, meaning it has a shining, lustrous appearance. While other minerals like pyrite also have a metallic luster, gold’s is especially brilliant. The luster of gold is described as shining to brilliant. The intensity of luster can help identify if a mineral is gold or not.
Hardness
Gold is relatively soft compared to other metals. On the Mohs hardness scale, gold has a hardness of 2.5 to 3. It can be cut and shaped easily and does not break or chip under pressure like many brittle minerals. Gold’s softness makes it effortless to scratch. You can test hardness by scratching an unknown mineral against a surface such as glass or unglazed porcelain. Gold will leave a visible streak. A knife blade or nail can also be used to attempt scratching the gold.
| Mineral | Hardness |
|---|---|
| Gold | 2.5 – 3 |
| Pyrite | 6 – 6.5 |
| Quartz | 7 |
Malleability
Gold is extremely malleable meaning it can be hammered, pressed, or squeezed into thin sheets without breaking. Gold leaves can be beaten down to only 0.0001 cm thick. Gold’s malleability makes it perfect for shaping into jewelry or gold foil. You can test malleability by gently hitting a gold mineral with a hammer to see if it flattens or by pressing the mineral between your fingers. Gold will be highly malleable while minerals like pyrite will shatter under a blow from hammer.
Density
Gold has a very high density compared to most other minerals. Gold has a density around 19.3 g/cm3 which is over 3 times the density of pyrite. High density is a useful identifying factor for gold. Testing density can be done by weighing a gold mineral on a scale. However, it is preferable to use water displacement methods. Submerge the mineral in water and measure the displacement volume. Divide the weight by the displacement volume to find density. An exceptionally high density reading indicates the likely presence of gold.
Magnetism
Gold is not magnetic and will not respond to the presence of a magnet. It does not become attracted to magnets and will not be deflected by magnetic fields. This is a useful test for distinguishing gold from magnetic pyrite. Passing a gold mineral near a magnet will result in no attraction. The lack of magnetism is a definitive test that can identify real gold.
Electrical Conductivity
Gold is the most electrically conductive metal, even more so than copper. It has high electrical and thermal conductivity ratings. Testing the conductivity of a mineral can therefore help identify gold. This can be done simply by touching the mineral with a metal wire attached to a circuit to see if it completes the circuit and turns on a tester light. Gold will have exceptional conductivity. Circuit testers are a common and readily available tool for distinguishing gold from non-conductive minerals.
acid Testing
Applying acids onto an unknown mineral sample can also help distinguish gold from other minerals. Gold is insoluble in nitric acid while pyrite will bubble vigorously. A few drops of nitric acid onto a sample will confirm that a mineral is real gold if there is no reaction observed. This test should be done carefully by putting a small amount of nitric acid on a crushed part of the mineral.
Flame Testing
Heating gold with a blowtorch will cause it to turn to a glowing yellow color. Pyrite on the other hand will become ash gray or black when heated. Using a flame test on a mineral sample can identify the presence of gold through its
characteristic incandescence.
X-ray Testing
More advanced gold detection techniques involve using x-rays to determine a mineral’s internal crystalline structure. Since gold has a very unique atomic structure, x-rays can be used to positively identify gold and distinguish it from other minerals. X-ray devices bombard minerals with x-rays and analyze the diffraction patterns that result. The diffraction pattern form gold is completely unique, allowing it be conclusively identified.
Extraction of Gold
Once gold is detected in rocks or mineral samples, extraction becomes necessary to harvest the valuable metal. Here are some of the main ways gold is extracted from ancient rocks and minerals:
Cyanide Leaching
Cyanide leaching uses cyanide salts to extract gold from finely crushed rock samples or ore. The cyanide binds to microscopic gold particles, separating them from waste rock. The cyanide-gold compound is then recovered via filters.
Mercury Amalgamation
Mercury is used to extract gold from crushed rock or ore by forming an amalgam – a gold-mercury compound. The amalgam is then heated to extract the gold from the mercury.
Froth Flotation
In froth flotation, powdered ore is placed in water containing surfactants. Air bubbles are blown through the solution. Gold particles become hydrophobic and attach to the bubbles which can be skimmed off.
Heap Leaching
Gold ore is crushed and piled in heaps. A cyanide salt solution is sprayed or dripped onto the heap over time which leaches out the gold. The solution containing the dissolved gold is collected.
Gravity Separation
Various gravity separation techniques utilize gold’s high density to isolate it out from low density waste rock. Methods include gold panning, shaker tables, jigs and more.
Conclusion
There are many distinctive properties of gold that allow it to be identified and detected in mineral samples. By testing characteristics like color, streak, hardness, malleability, magnetism and more, the presence of gold can be confirmed before extraction processing is begun. Advanced methods like x-ray diffraction can also conclusively identify gold. Being able to recognize real gold and distinguish it from look-alike minerals is an important skill for gold prospectors and collectors.