Niobium (element)
鈮 is a rare kanji used for the chemical element niobium. Its meaning is limited to this specific element name.
鈮 is a modern coined character for the element niobium, combining the metal radical 金 with the phonetic component 尼 (ni). It was created to represent the element's name phonetically.
The metal radical 金 hints at a metallic element, and the right side 尼 (ni) suggests the sound. Together they point to niobium, a metal named after Niobe.
For デイ, imagine a day when a new metal was discovered: day -> デイ, and that metal is niobium (鈮).