Why do some gemstones change their color
Various influences (light (photosensitivity), radioactive radiation, heat) can cause minerals to change their color.
Some minerals, such as rose quartz or fluorite, can fade when exposed to light. Chemical transformations can be triggered or accelerated by light (realgar -> auripigment), which is often associated with color changes. An example is intended to illustrate how complex the processes can be: Zendern wood oil is often rubbed into emeralds as a collector's crystal, which leads to a reinforcement of the color (and an improvement in transparency and shine) by filling cracks. However, UV light (contained in natural light) decomposes the cedar oil and released oxygen then bleaches the emerald (over a period of years).
Radioactive radiation can, for example, cause lattice defects that can act as color centers. For example, quartz (as smoky quartz) or fluorite (as stinkspar) shows such color changes in the (paragenetic) seam of uranium minerals. Artificially caused discoloration by radioactive irradiation is considered a counterfeit, but is widespread in the mineral and gemstone trade. Examples:
- Rock crystal -> smoky gray to black -> smoky quartz
- light topaz -> blue -> blue topaz
- Beryl -> yellow / green -> noble beryl
- Diamond -> yellow / blue / green -> rare diamond colors
- Tourmaline -> red / green -> stronger colors
There are different types of radiation
- high-energy electromagnetic radiation (beta, gamma)
- Neutron radiation, proton radiation
used, whereby (only) the latter can make the stone radioactive at the same time (which may be harmful to health when wearing such stones). The trade in irradiated stones has increased enormously in recent years.
Resistant color changes can also be brought about by heat. Brazilian amethyst turns light yellow at 470 ° C and dark yellow (citrine) to red-brown at 550 ° C to 560 ° C. Some smoky quartz can also take on the citrine color at 300-400 ° C. Some carnelians are burnt inferior chalcedony; today, agates are dyed, fired and irradiated with extreme to unnaturally colorful counterfeits on the market.
Color change due to interaction between chromophores and rare earths (REE)
The green color of some apatites (e.g. apatite from Panasqueira, Portugal) from green to colorless is the result of the interaction between the green-producing chromophore (Fe2+) and the REE concentration (Nd3+, Ce3+). The higher the REE concentration, the more colorless the color zones become. It is noteworthy that these REE-enriched zones are the zones with the strongest luminescence. This discoloration mechanism is attributed to the oxidation of the chromophore or to additional absorption. (In the glass industry, Nd3+ and Ce3+ used as an absorber to discolor green glass or to weaken the green color).
Color, color zoning, absorption, color center
Sources / Links:
- Knutson, C., Peacor, D.R., Kelly, W.C., 1985; Luminescence, color and fission track zoning in apatite crytslas of the Panasqueira tin-tungstendeposit, Portugal. At the. Min. Vol. 70, 829-837 (color change due to interaction between chromophore and REE).
- Institute for Gem Testing
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