Gold is a precious metal that has captivated humankind for thousands of years. It has been highly valued for its rarity, physical properties, and its use as currency. When thinking about gold, most picture the brilliant yellow color associated with pure gold. However, gold in its natural state does not always appear yellow. When mixed with other minerals and materials in the earth, gold can take on different hues and colors based on its chemical composition and surrounding geology. This article will examine what makes gold its signature golden yellow, what other colors it can appear in the earth, and the science behind gold’s chameleon-like color variations.
What Makes Gold Yellow?
Pure metallic gold is yellow in color when it reflects light. This is because of the unique properties of gold’s electrons and how they interact with visible light wavelengths. Specifically, there are two key factors that give gold its yellow luster:
– Gold’s electron configuration – The arrangement of electrons in gold’s atoms resonates strongly with blue light wavelengths. This resonant scattering of blue light makes the reflected light appear yellow.
– Relativistic effects – Gold contains heavy atoms with many protons in the nucleus. This contracts the orbitals of valence electrons closer to the nucleus, causing the electrons to move faster. According to special relativity, this causes the electrons to increase in mass, changing how they interact with light. This also contributes to gold’s yellow color.
These intrinsic atomic properties of gold make it uniquely golden in color when in its pure form. However, when gold is mixed with other metals and minerals, its color can shift dramatically.
Colors of Gold in the Earth
When gold is found naturally, it is rarely pure. Gold often contains silver, copper, zinc, and other metals mixed into the natural alloy. Additional minerals present in the surrounding ore and rocks will also impact gold’s hue. Based on these factors, gold can take on many colors beyond just yellow. Some of the most common colors of gold found in nature are:
Yellow Gold
Yellow gold indicates a higher purity of gold. Jewelry is often ranked based on karatage, which measures gold purity on a 24-part scale. 24 karat gold is 100% pure gold, while lower karatages contain more alloyed metals. So the most brilliant, intense shades of yellow gold signify the highest gold purity.
White Gold
White gold usually contains 25% nickel and zinc. The addition of these white metals turn the gold a silver-white color. White gold is commonly used for jewelry and decorative arts. The color can also resemble very pale yellow, depending on the precise metal mixture.
Rose Gold
Rose gold takes on a pinkish hue from the addition of copper. Typical rose gold contains 75% gold and 25% copper, although the ratio can vary. The more copper added, the redder the gold appears. Rose gold has surged in popularity for jewelry and fashion accessories in the 21st century.
Green Gold
Natural green gold is rare and forms when silver oxidizes near gold deposits. The oxidization causes the silver to turn greenish, tinting nearby gold as well. Jewelers also intentionally add silver and sometimes cadmium to gold to produce the popular green gold jewelry metal.
Blue Gold
Natural blue gold is found when gold contains traces of iron. When iron oxidizes, it can turn gold blue-green. Jewelers often add iron and cobalt salts during alloying to intentionally produce vivid blue gold jewelry.
Purple Gold
Natural purple gold occurs when gold contains trace amounts of manganese, aluminum, or titanium. Jewelers can also create purple gold by adding aluminum to the alloy. The more aluminum added, the more purple the final color.
Black Gold
Black gold is created when gold is alloyed with cobalt. The higher the cobalt content, the darker the gold appears. Black gold is a popular jewelry metal thanks to its striking coloration.
What Determines Gold’s Color in Dirt?
Now that we’ve examined the spectrum of gold colors, what makes gold appear one hue versus another when found in the earth? Here are the primary factors that determine the color of raw gold in dirt and rock ore:
1. Surrounding Minerals
The types of minerals adjacent to gold deposits directly impacts gold’s hue. If silver is present, green gold can form. Iron can turn gold blue-green. Copper may make gold appear more pink or rose-colored. The minerals available will alter reflective properties and oxidation states of the gold.
2. Oxidation
Oxidation occurs when gold is exposed to oxygen and other compounds. This causes electrons to move between minerals and metals, altering their chemical structure. The oxidation process can change minerals like silver and iron into greenish or blue hues, tinting nearby gold.
3. Percentage of Alloy Metals
The more metals like copper, silver, nickel, and zinc mixed into gold, the more its color will diverge from pure yellow. Alloy percentages are the key driver behind colored gold like white, rose, and black. The more alloyed metals, the more dramatic the color change.
4. Trace Elements
Tiny amounts of metals like manganese, titanium, aluminum, and cobalt can impact gold colors too. As little as 1% of these in gold can turn it purple, blue, or black depending on the element.
Evaluating Gold Colors in Dirt
Now that we understand the science behind gold’s chameleon-like color variations, how can miners and prospectors evaluate gold colors in the field? Here are some tips for analyzing gold hues found in dirt:
1. Understand the Geology
Research the geology and mineral content of a gold deposit. Understanding common adjacent minerals helps explain gold colors. For example, areas with high iron content are more likely to have blue-green gold. Knowing the geology provides context.
2. Examine Ore Streaks
Crush samples of gold ore into powder and examine streak colors. This tests for other minerals mixed with gold. Pink, black, white, or green streaks suggest hues that may tint gold findings.
3. Assess Karatage
Evaluating gold karatage reveals purity levels. Jewelers often test karatage with acids like nitric acid. But in the field, visual color analysis can provide a quick karatage guide. More golden yellow suggests higher purity, while paler gold has more diluted alloy metals.
4. Consider Oxidation Patterns
Note surface oxidation patterns on gold deposits. Does oxidation appear localized or widespread? This helps determine if oxidation is changing gold’s color broadly versus just surface effects.
5. Look for Banding
Examine gold ore samples for banding – discrete color variations layered in the ore. Banding indicates zones of different mineral mixtures, which alters local gold colors in that section. This creates a striped effect in raw gold ore.
Conclusion
While pure gold is universally recognized for its yellow brilliance, natural gold takes on a rainbow of hues when mixed with other metals and minerals. Factors like adjacent geology, oxidation, alloy percentages, and trace elements combine to give gold an intriguing diversity in coloration. Understanding the science behind gold’s varied pigments equips miners and geologists to evaluate gold discoveries in the field. So while gold itself may not change, its distinctive colors provide insight into the diverse composition of ores and mineral deposits found deep in the earth.