Grobnite

Grobnite is only mentioned once in Metroid Prime 2 Echoes and throughout the whole prime trilogy. While at the Sanctuary Entrance, the player can scan a huge rectangular block of glass that guards access to an elevator. Using scan visor, the contents of the glass are said to contain denzium and grobnite. Since this is the only game that grobnite appears in it's safe to say that grobnite may be a glass like element (assuming that the denzium is mixed in with the grobnite). Grobnite is not indestructible however, while despite containing denzium which can be destroyed with power bomb, the grobnite glass is cracked by space pirates (Commandos assumably, since they were located at the sanctuary entrance) when Samus Aran arrives at sanctuary fortress.