Uncommon words:
General:
* Ephemeral: Lasting for a very short time.
* Serendipitous: Occurring or discovered by chance in a happy or beneficial way.
* Ubiquitous: Present, appearing, or found everywhere.
* Mellifluous: Sweet or musical; pleasing to the ear.
* Quixotic: Extravagantly idealistic; unrealistic and impractical.
* Petrichor: The pleasant, earthy scent after rain.
* Sesquipedalian: Using long words.
* Ephemeris: A table showing the daily positions of celestial objects.
* Supercilious: Behaving or looking as though one thinks one is superior to others.
* Flibbertigibbet: A frivolous, flighty, or eccentric person.
Nature:
* Efflorescence: The state of flowering.
* Crepuscular: Relating to twilight or dusk.
* Pansy: A variety of violet with showy flowers.
* Lichen: A composite organism that grows on rocks, trees, etc.
* Dendrochronology: The study of tree rings.
* Mycelium: The network of fungal threads that forms the main body of a fungus.
Literature & History:
* Pastiche: A literary, musical, or artistic work that imitates the style of another work, author, or artist.
* Cadence: The rise and fall of the voice or music.
* Scintilla: A tiny spark or trace of something.
* Missive: A letter, especially a formal or official one.
* Grandiloquent: Using high-flown or bombastic language.
* Convivial: Relating to feasting, drinking, or merrymaking.
Other:
* Onomatopoeia: The formation of words from sounds associated with what they represent.
* Juxtaposition: The fact of two things being seen or placed close together with contrasting effect.
* Synecdoche: A figure of speech in which a part is used to represent the whole or the whole to represent a part.
* Haptic: Relating to the sense of touch.
* Loquacious: Tending to talk a great deal; talkative.
Note: This list is not exhaustive, and the "uncommonness" of a word can vary depending on context and individual familiarity. Many words can be considered uncommon depending on the specific audience and situation.
You can also explore online resources like dictionaries and thesauruses to find more uncommon words.