Garnet is a gemstone known for its rich, deep red color. However, some garnets can display a range of colors from greens to oranges to pinks. So why does garnet sometimes change colors? The reasons have to do with the mineral composition and structure of different types of garnet.
What gives garnet its red color
The most common type of garnet is almandine garnet which has a deep red color. This red color comes from the presence of iron and aluminum in its mineral structure. Specifically, it is the iron ions in the crystalline structure of garnet that absorb certain wavelengths of light and reflect back the red wavelength to our eyes. The more iron present in the garnet, the deeper and richer the red color.
Reason 1: Different chemical compositions
While almandine garnet has a high iron content, other types of garnet contain different amounts and ratios of metal ions like magnesium, calcium, chromium, and manganese. These differences in mineral composition affect the absorption and reflection of light, leading to variations in color.
For example, pyrope garnet tends to be more of a purple-red because it contains more magnesium and less iron. Demantoid garnet can display vivid greens because it has chromium. Rhodolite garnet contains manganese giving it a pinkish-red hue. The lack of iron in many garnet types is why they do not display that rich ruby red.
Reason 2: Small trace elements
Even within the same type of garnet, small differences in trace elements can impact color. A tiny change in iron, chromium or manganese content in pyrope garnet for instance can shift the shade from lightly purple to deeply purple. Environment conditions like heat or radiation exposure can sometimes alter the structure enough to modify color slightly too.
Reason 3: Crystal size
The size of the individual garnet crystals also affects the absorption and reflection of light. Smaller crystal sizes tend to allow more light to pass through, reducing the intensity of color. Larger crystal sizes reflect more red wavelengths, creating a deeper red tone.
Garnet crystals over 2 cm in size typically display the most saturated and vivid colors. The largest almandine garnet crystals can appear almost black they are so darkly red. Meanwhile, small finely crushed garnet crystals used in industrial abrasives take on a lighter red-brown color.
Different Types of Colored Garnets
Here is an overview of some common garnet varieties that display colors besides red:
| Type | Color | Composition |
|---|---|---|
| Tsavorite | Green | Calcium aluminum garnet with vanadium and chromium |
| Demantoid | Green | Calcium iron garnet with chromium |
| Mali | Yellow-green | Calcium iron garnet with chromium |
| Rhodolite | Purple-red | Magnesium aluminum garnet with manganese |
| Malaya | Pink to light red | Calcium aluminum garnet with manganese |
| Spessartite | Orange | Manganese aluminum garnet |
| Hessonite | Orange-brown | Calcium aluminum garnet with manganese |
| Grossular | Yellow, green, pink, red | Calcium aluminum garnet |
| Andradite | Yellow, green, brown, black | Calcium iron garnet |
| Uvarovite | Green | Calcium chromium garnet |
Environmental causes of color change
In some cases, garnet can experience natural color shifts during its geological formation or after being mined. Environmental conditions like radiation exposure, high temperatures, or mineral reactions can modify trace elements or the crystalline structure enough to cause color changes. Common causes include:
- Heating – Higher temperatures can remove trace elements like chromium or alter crystal structures. This can shift green garnets to more brownish tones.
- Radioactivity – Exposure to radiation can reduce iron ions, modifying the red color to more yellow or brown.
- Oxidation – Iron ions in garnet react with oxygen, decreasing redness.
- Dehydration – Water loss transforms calcium garnets from blue/purple to more green.
These kinds of environmental shifts in color tend to be relatively minor unless exposure is extreme or prolonged. In general, most garnets exhibit good color stability and only change minimally over time.
Color change garnets
There is a very rare type of natural garnet that exhibits dramatic color changing ability. These garnets can shift from blue-green in daylight to purple-red in incandescent light. Only a small number of garnet specimens from Madagascar display this remarkable color change property.
The exact cause of the color change remains unclear but is thought to involve vanadium ions within the garnet’s structure. Something about the valence states or coordination of the vanadium allows different wavelengths of light to be absorbed or reflected in different lighting conditions, creating the dramatic color shift.
Conclusion
Garnet gets its signature red color from iron but other elements like magnesium, chromium, and manganese can give garnet a wide spectrum of colors from green to purple to orange. Small differences in these trace elements along with crystal size and environmental factors all contribute to the variations in garnet color. The remarkable ability of color change garnets remains somewhat mysterious but provides a stunning example of how garnet can transform.
For centuries, the red color of almandine garnet has made it a popular gemstone. But as other garnet types were discovered that expanded the color possibilities, they have become increasingly valued. The range of colors now available among the many garnet varieties and sources allows this gemstone to offer something for everyone.
