Handbook of Mineralogy · Quartz
Cite: Handbook of Mineralogy (Mineralogical Society of America). Citrine is a variety of quartz (SiO2), not a separate IMA species. https://www.handbookofmineralogy.org/pdfs/quartz.pdf
Mineral
Citrine · yellow quartz
Citrine is yellow to amber quartz, a variety of SiO2, not a separate species (Webmineral quartz page). Habit follows the host: hexagonal prisms with pointed terminations when the piece is crystalline, or massive when it was cut from a core. Mohs hardness is 7, the same scratch test as colorless quartz. GIA states that yellow quartz is rare in nature and that most citrine is the result of heating, which converts purple amethyst to golden shades. That is a trade fact, not a species split. People collect both. Cards below cite the quartz species pages. See Quartz varieties.
Cited public sources
Webmineral · Quartz varieties
Cite: Webmineral Quartz Mineral Data. Lists citrine as yellow quartz. Named species page. https://webmineral.com/data/Quartz.shtml
GIA · treated citrine
Cite: Gemological Institute of America. Most citrine is heated amethyst. https://www.gia.edu/gia-faq-treated-citrine