"Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self-contained, and solitary as an oyster..."
See in text (Stave One)
These two similes define Scrooge in three ways: First, he is portrayed as inflexible through the comparison to flint (a hard gray rock). Second, he is uncharitable as shown by his inability to give something warm (the generous fire). Finally, he is not only isolated from others, but he also keeps to himself in his own world, contained within his own shell.