Valentinite is an antimony oxide mineral with formula Sb2O3. Valentinite crystallizes in the orthorhombic system and typically forms as radiating clusters of euhedral crystals or as fibrous masses. It is colorless to white with occasional shades or tints of yellow and red. It has a Mohs hardness of 2.5 to 3 and a specific gravity of 5.6 to 5.8. Valentinite occurs as a weathering product of stibnite and other antimony minerals. It is dimorphous with the isometric antimony oxide senarmontite.

A mineral named in the middle of the 19th century in honour of Basilius Valentinus, a writer on alchemy, whose identity is rather a problem. There are considerable difference of opinion as to when exactly he was supposed to have lived (16th or 17th centuries). But he is the supposed author of the first book to give a detailed description of antimony and its compounds. From the contents of the book it is also obvious that Valentinus was familiar with the synthetic preparation of antimony trioxide, which was called 'the antimony flower'.

Valentinite was first discovered near Allemont in France towards the end of the 18th century; the first description of its occurrence in the region of Pribram in Bohemia comes roughly from the same time. This particular locality at one time produced the very best crystals of this mineral. The largest crystals found there measured up to 3 cm. Grouped in ruch druses, they developed in vein cavities with galena. Such specimens were very rare and much valued by collectors. Each crystal was worth a gold ducat. The renowned Belsazar Hacquet (1739-1815), who was born in France, trained as a doctor and then became a brilliant geologist who centred his attention on the mineral world of central Europe, complained in one of his treatises that he actually had to pay two ducats for a single crystal of valentinite from Pribram. Valentinite is a weathering product of antimony veins, where it forms as a seondary mineral through oxidation in the upper parts of the deposits. It forms columnar to needle-shaped crystals. A rich deposit of valentinite has been found only in one locality- the Constantine province of Algeria. This is the only deposit where it is mined as an ore, with 83% antimony. In all other locations it occurs in negligible quantities.