Many unscrupulous dealers sell amethyst which has been heat treated as citrine. Citrine is rare, very rare.
So let's set the record straight, amethyst is purple Quartz. Smoky Quartz can be heat treated also. My reading suggests heat treated amethyst is what most dealers sell as citrine. A reputable dealer will tell you. I am looking for references to heat treated smoky wuartz and will update this if I find anything significant.
So, after a review of some of the available data, smoky Quartz is also heat treated to form a citrine colored stone.The smoky Quartz I have collected has the smoky black/green in it already. And, While the coloration in Citrine may be due to heat treating not all stones are the same, so my answer is some smoky Quartz has the green hue in it. That is the color you get when you mix black and yellow. The color of the stone may indicate smoky quartz but it is not necessarily definitive.