Tungsten (element), Wolfram
鑕 centers on the metallic element tungsten (also known as wolfram). It is a specialized character for this heavy, high-melting-point metal.
鑕 is a phono-semantic compound: 金 (metal) provides the meaning, and 質 likely contributes the sound. It was created as a character for the element tungsten.
The metal radical 金 on the left marks this as a metallic element. The right side 質 suggests a dense, substantial quality, fitting tungsten's heaviness.
For シツ, imagine a sheet of tungsten metal: sheet -> シツ, and the heavy metal sheet is made of tungsten.