I constantly see the debate on whether heat treated Amethyst is also Citrine since they are from the Quartz species.
Citrine can be confused with Topaz but often, heat treated Amethyst or synthetic glass has been marketed as Citrine due to its rarity and a higher price tag.
There are several colour variations for Citrine and marketed under several names including Lemon Quartz (yellow with a green tint), Golden Citrine (saturated yellow), Madiera Citrine (orange-red and brown) or Madeira Citrine (yellow with a bright orange tint).
It is a relatively simple process for Amethyst and Smoky Quartz to produce the yellow colour. Amethysts are heated to 250-500 degrees Celsius by being baked in an oven or buried in sand and heated for several hours until the desired colour is achieved. Another way is to initially use gamma radiation on Clear Quartz to form Smoky Quartz which is then heat treated.
Quartz being heated..isn't it still the same thing then?
Well, Citrine grows in individual crystal points and very small clusters, not large geodes which is what Amethyst tends to grow in.
Amethyst will often twin poly-synthetically where it has an inter-growth of right- and left-handed Quartz. Under polatised light, a Brewster's fringe which are dark bands in a symmetrical triangular pattern can be seen down the C-axis (runs from one end of the crystal to the other). This can be a way to show that heat treatment may have occurred if a Citrine piece has a Brewster's fringe.
Natural Citrine is dichroic with 2 shades whereas pleochroism is not present in heat-treated Amethyst, except Lemon Quartz (heat-treated Smoky) where it has slight dichroism of yellow-green and yellow. Pleochroism is when something observed at certain angles will have different colours.
The yellow colour in Citrine is caused by a few ways. The first is natural irradiation of Quartz in the ground and the presence of Aluminium in its structure. Natural irradiation causes a hole colour centre. Aluminium then absorbs light from irradiation to form yellow. Another way is the presence of iron oxide - Hematite or Goethite. A charge can be transferred between Fe3+ and O2- which results in an absorption and the yellow colour.
So...in a nutshell, Citrine is not the same as heat-treated Amethyst. Citrine displays diochroism, has a hole colour centre from natural irradiation and the presence of Aluminium in it's chemical structure which produces the yellow colour. HTA instead produces the yellow colour via an iron colour centre and colour formation is man-manipulated.
Regardless, I don't mind either, natural or heat-treated.