On her website, Rowling wrote that Voldemort's wand is made of yew, whose sap is poisonous, and which symbolises death. It forms a deliberate contrast to Harry's wand, which is made of holly, which she chose because holly is alleged to repel evil.
on Her webSiTE, RoWLinG WrotE That voldeMorT's wAnd Is maDe of yew, WHOSe sAP is poIsOnous, and Which sYMBoliSes DeAtH. It FoRms A DelibEratE CoNtraST to HarRy's WANd, whIcH is MAde of HolLY, which sHe chose because hoLLy is aLLeGEd TO repel EviL.