Natural citrine is one of the most prized varieties of the mineral quartz.
Pale yellow coloured stones are readily available. They're often heated to mimic other varieties of quartz.
Deep yellowish-orange coloured citrine is highly sought after. Yellowish brown or brownish orange is also popular and is generally more affordable. This material is often used for tumbling.
The vast majority of citrine being sold is amethyst that has been heated. It's usually quite easy to tell natural citrine from heated amethyst.
Citrine is the birthstone for November on the modern birthstone chart.