Lillie said:
I have a citrine crystal....one that was nice and dark yellow....Well now it isn't. It's faded....Do they all do that? ....Is it possible that it will ever 'un-fade'? Oh yeah. And it has got maroon spots on it.
Your citrine was heat treated, and not done very well either, and now it's fading and going through another transformation - but don't worry - it could become a really striking beauty!
Most dark yellow or orange citrine found on the market is purple amethyst that has been heat treated in a heating chamber to make it turn yellow. Actually, even yellow citrine straight from the earth is amethyst that changed color from heat within the earth. But natural citrine from the earth is a more natural shade of yellow. Sometimes crystals are dug up from the earth that are part yellow and part purple and then they are known as Ametrine (a very powerful healer !!!)
Those maroon spots in your stone are mineral inclusions of raw iron or manganese, and possibly even some titanium as well. They are what gives the amethyst crystals their violet and purple colors - the colors can 'bleed' through the stone like a wash of water color and stain the stone purple. Once heated at very high temperatures the manganese and other inclusions fade and bleach out, leaving the stone orange or an unaturally bright yellow.
The bright yellow isn't going to come back now, not even re-heating it will bring it back. But if you soak the stone in water for several days the remaining maroon spots may begin to bleed their purple color or a purple-brown color back into the stone. Look at them with a high powered magnifying glass and you may see what looks like little threads of color growing outwards from the spots. Those threads might even grow and become like little sun-bursts of gold colored rutile (I have one here at home doing that right now). If the stone becomes part violet and part yellow it will be ametrine and you'll have a highly desirable healing stone in your possession.
Trismegistus - I apologize for contradicting you but ALL quartz crystals are capable of changing their color naturally or of being changed by man-made means. ALL quartz will fade under excessive sunlight or bright hot electric lights, and amethyst and rose quartz are the most susceptible to bright light or heat.
There are many, many other stones besides quartz that will break down or change color or bleach out from exposure to bright light or heat - and iron pyrite (fools gold) will actually vaporize into sulphuric acid.