“Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble;” (1Co 3:12, KJV)
I suggest that the meaning of those building materials will be found within the chapter.
What is Paul addressing in this chapter?
“And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able.” (1Co 3:1-2, KJV)
Paul is addressing babes in Christ. Their spiritual level is a restriction to what and how Paul was going to address them. Due to their spiritual quality, Paul had to address them as though they are not saved: as people of the world speak. He spoke to them as unto carnal. Paul went on to point out their carnality vis-a-vis envy, strife, divisions, party spirit. Hence, I suggest that this chapter focuses on there carnality and what produced these carnal behaviours. This is the light from which to see the metaphors used by Paul.
“For while one saith, I am of Paul; and another, I am of Apollos; are ye not carnal? Who then is Paul, and who is Apollos, but ministers by whom ye believed, even as the Lord gave to every man?” (1Co 3:4-5, KJV)
It seems to me that there were those who became saved via Paul's message as well as those who became saved via Apollos' message. There are two camps, based on who led them to Christ, within the church in Corinth. Paul says that they are "ministers by whom ye believed."
“I have planted, Apollos watered; but God gave the increase.” (1Co 3:6, KJV)
In verse 9, Paul says "...ye are God's husbandry...". Paul planted and Apollos watered on God's husbandry. God's husbandry are belivers. They are already in the kingdom. Paul planted and Apollos watered the beielievers.
In using another metaphor, Paul described these belivers as God's building vs 9. Henceforth, from verse 10 , Paul elaborated on these believers using the metaphor of a building. If these believers are a building, they are the different material products produced by Paul and Apollos that make up the building.
Hence, some believers in this church have been produced as gold, silver, precious stones, wood, hay and stubble. Again, this is just a metaphor to describe the believers in Corinth.
What made up these end-products of believers?
To answer this, the metaphor of farming and husbandry will have to be looked at again. Paul planted, Apollos watered these believers. vs 7.
It seems to me that what they taught these believers when each of them visited them produced the quality of believers, especially the carnal ones that Paul is dealing with here. They are the products of doctrines, teachings, instructions, corrections etc they received from the labourers. Doctrines, teachings, instructions, corrections, motivations, encouragements they taught fall into the categories of gold, silver, precious stones, wood, hay and stubble.
It seems to me that every believer is a product of what they have been taught! Those taught gold-like teachings become like gold. Those taught wood-like teachings become like wood in quality and usefulness and so on.
“If any man's work abide which he hath built thereupon, he shall receive a reward. If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.” (1Co 3:14-15, KJV)
Just as these teachers who taught wood, hay and stubble quality teachings "...shall be saved; yet so as by fire." The believers who have received such teachings also, "...shall be saved; yet so as by fire."
The carnality shown by the believers in Corinth is a product of what sort of teachings they have received.
There is reward for what sort of teachings, doctrines and etc we churn out to God's building!
How do we and what do we plant in the hearts as well as how and what kind of water do we water God's husbandry?