Guaiacum (//), sometimes spelled Guajacum, is a genus of flowering plants in the caltrop family Zygophyllaceae. It contains five species of slow-growing shrubs and trees, reaching a height of approximately 20 m (66 ft) but are usually less than half of that. All are native to subtropical and tropical regions of the Americas and are commonly known as lignum-vitae, guayacán (Spanish), or gaïac (French). The genus name originated in Maipurean, the language spoken by the native Taínos of the Bahamas; it was adopted into English in 1533, the first word in that language of American origin.
The genus is famous as the supplier of Lignum vitae, which is the heartwood of several species in the genus. It is the hardest wood that is measured using the Janka hardness test, requiring a force of 4,500 lbf (20,000 N) to embed a steel ball 0.444 in (1.13 cm) in diameter a distance half of that into the wood.
The Spanish encountered guaiacum wood "when they conquered San Domingo; it was soon brought back to Europe, where it acquired an immense reputation in the sixteenth century as a cure for syphilis and certain other diseases. 
Gum from the wood was used to treat syphilis; for example, Benvenuto Cellini records this use of it in his memoirs. Thomas Nashe refers to its supposed medical properties in his tract Nashe's Lenten Stuff, as well as to the exotic sound of the word at the time (playing on the famously bizarre-sounding Latin word Honorificabilitudinitatibus, meaning "worthy of honour"): "Physicians deafen our ears with the Honorificabilitudinitatibus of their heavenly Panacaea, their sovereign Guiacum." Guaiacum resin has been used to treat a variety of medicinal conditions from coughs to arthritis. Wood chips can also be used to brew a tea.
The artist Jan van der Straet (also known as Johannes Stradanus) painted a scene of a wealthy man receiving treatment of syphilis with Guaiacum wood sometime around 1580. The title of the work is "Preparation and Use of Guayaco for Treating Syphilis." Epidemic syphilis had been raging through Europe for nearly a century at the time of the painting, and hopes were high that this plant from the New World would provide a cure. The richly colored and detailed work depicts four servants preparing the concoction while a physician looks on, hiding something behind his back while the hapless patient drinks.
The 1955 edition of the Textbook of Pharmacognosy also says that: "Guaiacum has a local stimulant action which is sometimes useful in sore throat. The resin is used in chronic gout and rheumatism, whilst the wood is an ingredient in the compound concentrated solution of sarsaparilla, which was formerly much used as an alterative in syphilis."
A phenolic compound derived from the resin of Guaiacum trees is used in a common test for blood in human stool samples. The presence of heme in the blood causes the formation of a coloured product in the presence of hydrogen peroxide. The effect of peroxidases in horseradish on guiacum was first noted in 1810 by Planche.
- Guaiacum angustifolium Engelm. – Texas Lignum-vitae (Texas, Northeastern Mexico)
- Guaiacum coulteri A.Gray – Sonoran Lignum-vitae (Western Mexico, Guatemala)
- Guaiacum officinale L. – Common Ligum-vitae (The Caribbean, Northern South America)
- Guaiacum sanctum L. – Holywood Ligum-vitae (southern Florida, The Bahamas, Southern Mexico, Central America, Greater Antilles)
- Guaiacum unijugum Brandegee (Northwestern Mexico)
Formerly placed here
- Porlieria microphylla (Baill.) Descole et al. (as G. microphyllum Baill.)
- Schotia afra (L.) Thunb. (as G. afrum L.)
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- "Genus: Guaiacum L". Germplasm Resources Information Network. United States Department of Agriculture. 2008-05-20. Retrieved 2009-09-25.
- "Guaiacum L". TROPICOS. Missouri Botanical Gardens. Retrieved 2009-12-06.
- OED 2nd edition, 1989.
- Entry "guaiacum" in Merriam-Webster Online Dictionary, retrieved 2013-04-30.
- Grandtner, Miroslav M. (2005). Elsevier's Dictionary of Trees: With Names in Latin, English, French, Spanish and Other Languages 1. Elsevier. pp. 389–391. ISBN 978-0-444-51784-5.
- Bailey, Richard W (2004). "Part I - American English: its origins and history". In Edward Finegan; John R. Rickford. Language in the USA: Themes for the Twenty-first Century. Cambridge University Press. p. 3. ISBN 978-0-521-77747-6.
- Gordon, J. E.; González, M. A.; Vázquez Hernández, J.; Ortega Lavariega, R.; and Reyes-García, A. (2005). "Guaiacum coulteri an over-logged dry forest tree of Oaxaca, Mexico". Oryx (Fauna & Flora International) 39 (1): 82–85.
- "National Symbols". Emancipation & Independence. Jamaica Information Service. Retrieved 2009-12-06.
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- Self, Charlie (2005). Woodworker's Pocket Reference: Everything a Woodworker Needs to Know at a Glance. Fox Chapel Publishing. pp. 18–19. ISBN 978-1-56523-239-6.
- Wallis, Thomas E (1955). Textbook of Pharmacognosy.
- Johannes Stradanus undated brief review of works. Accessed August 6, 2007.
- Jan van der Straet's "Preparation..." at commercial art site. Accessed August 6, 2007.
- Azevedo AM, Martins VC, Prazers DM, et al. (2003). "Horseradish peroxidase: a valuable tool in biotechnology". Biotechnol Annu Rev. Biotechnology Annual Review 9: 199–247. doi:10.1016/S1387-2656(03)09003-3. ISBN 978-0-444-51400-4. ISSN 1387-2656.
- "Guaiacum unijugum Brandegee". Germplasm Resources Information Network. United States Department of Agriculture. 2006-05-19. Retrieved 2009-12-06.
- "GRIN Species Records of Guaiacum". Germplasm Resources Information Network. United States Department of Agriculture. Retrieved 2010-09-19.
- "Subordinate Taxa of Guaiacum L". TROPICOS. Missouri Botanical Gardens. Retrieved 2009-12-06.