According to I Corinthians 3:7-14, what principles are applied in the rewarding of stewards?

The stewards will be rewarded according to the effort they had invested in building on God's foundation (the only foundation one can build on). They will be rewarded according to the care they had shown, as well as the materials they had chosen to build from: gold, silver, stone, wood, hay, or straw. These materials have different features and the buildings made of them will not last equally. All of those things will be taken into account. Of course, St. Paul is talking in allegory here. He refers to all the pious and diligent people who spend their lives as God intended them to, and will be rewarded accordingly.