Alexandrite, sapphire, garnet, tourmaline, spinel, synthetic sapphire, glass etc. are some of the gemstones which display a change of color from daylight to lamplight.
To judge whether a stone is a color changing gemstone or not, one has to consider following things:
- First and foremost, always keep the light source constant such as the daylight, non-fluorescent tube and a lamp.
- The distance of the stone from the light source must be constant for both sources.
- The stone to be examined must be kept on a white background with the table facet towards the light source and observed with light reflected off it.
- If and only if there is a perceptible change of color from one source to the other, the stone can be considered as a color changing stone.
- The percentage of change however can only be an approximate idea of whether it is 50% or 75% or 100% unless predetermined with a master graded set. This factor is the deciding factor for calculating the price.
- The most important factor is that the stone is observed in reflected light and not transmitted light.
- Some stones are so strongly pleochroic (some have a secondary reddish / brownish tint), that in lamplight the color change is magnified and enhanced and the stone appears to have a strong color change when in actual fact it is only slightly enhanced.
- Colored inclusions such as orange-brown staining in fractures also tent to enhance the color of the stone.
These are some of the points that one should consider while examining a color changing gemstone, but it is always recommended to take the help of gemological instruments and lab.