Amanita gemmata, commonly known as the gemmed Amanita or the jonquil Amanita, is a mushroom of the genus Amanita, a genus of fungi including some of the most deadly mushrooms, as well as notably psychoactive mushrooms. Amanita gemmata resembles the false death cap, tawny grisette and panther cap mushrooms. Its pileus (cap) is a more bright color than in the former and more yellow than that of the latter two. Yet today it is still confused with various other European species.
The fruit bodies are colored yellow overall. The fresh cap, ranging in color from dull creamy yellow to golden yellow to buff, is sticky when moist. White warts adorn the cap surface, but they are usually flimsy and easily washed away by rain; they even seem as though they might easily slide off the wet cap surface with no more than gravity to encourage them. The cap is typically 4–10 cm (1.6–3.9 in) in diameter, initially convex before flattening out in maturity. The gills are adnate to adnexed, and white; they are close together, with little intervening space. The pale yellowish stem is 5–13 cm (2.0–5.1 in) long by 0.5–2 cm (0.2–0.8 in) thick, and either roughly equal in width throughout, or slightly thicker at the base. Young mushrooms have a membranous partial veil extending from the upper stem to the cap margin; as the mushroom grows, the partial veil tears to leave a flimsy, skirt-like, easily-lost ring on the stem. At the base of the stem is a flimsy white volva (a remnant of the universal veil that covered the immature mushroom) that usually forms a small, free rim. Spore prints are white.
Amanita gemmata has ellipsoid to broadly ellipsoid spores measuring 8–10 by 6.5–7.5 µm with an average Q-ratio (the fraction of length/width) of 1.35; the spores are not amyloid. The surface of the thin-walled spores is smooth, and they contains one to several small oil droplets. The basidia (spore-bearing cells of the hymenium) are mostly unclamped.
The species was implicated in several poisonings. Symptoms were similar to that caused by ingestion of Amanita phalloides, including acute gastroenteritis and acute hepatitis.
There are numerous forms in North America that tend to integrate with Amanita pantherina. In 2005, Rod Tulloss described Amanita aprica, a species that has been confused with Amanita gemmata several times in the past.