Although widely referred to as citrine, these natural rough crystals are in fact heated amethyst. Due to it being extremely rare, citrine is widely produced artificially by heating the purple variety of the mineral quartz. When heated to in excess of 500 °C amethyst changes colour. For many years most people were blissfully unaware that the vast majority of citrine was actually amethyst.
Distinguishing natural citrine from heated amethyst is relatively easy certainly in regards to most commercial grade material. The two most obvious clues are colour and price. Were it to be possible to find natural citrine crystals in this size you would pay an eye-watering price. Citrine which is the yellow variety of the mineral quartz is really quite rare.
Artificially produced citrine tends to have a rich yellowish orange colour and can even be almost brown. Natural citrine on the other hand has a very subtle yellow or golden colour. Although citrine have also be distinctively orange, the crystals would never include opaque white quartz.
It's worth remembering that many minerals are heat treated to enhance or change their colour and have been for thousands of years. In fact it's extremely unusual for a fine ruby or sapphire not to have been heated. Another interesting fact is the vast majority of citrine geodes are not natural citrine.
Despite having been heated these large and chunky 'citrine' crystals are exceptionally tactile and like most heated amethyst, come from Brazil. Individual weights and sizes can be found in the adjacent dropdown list. Number 1 is the citrine crystal on the left in all three photos. Number 2 is in the middle and number 3 is on the right.