Gold and money are the substitutes for love and companionship in the life of Silas Marner.
Silas was once a relatively happy man. He was well-known in his town of Lantern Yard, had a strong standing in his church, was engaged to be married, and enjoyed his life fully
His life, before he came to Raveloe, had been filled with the movement, the mental activity, and the close fellowship, which, in that day as in this, marked the life of an artisan early incorporated in a narrow religious sect.
Then, tragedy struck. Silas was framed by his so-called best friend, William Dane, and accused of robbery. The sadness that accompanied the accusation was made worse when Dane also takes Marner's fiancée from him, prompting Silas to leave town a broken man.
Once in Raveloe, he discovers that the grief of the events of Lantern Yard were superficially soothed by working long hours and collecting the fruits of his labor. To Silas, money was not as important in Lantern Yard. He used it for the typical comforts of any citizen who works for a living. Since none of the daily happenings that filled his life existed anymore, however, money and gold became the substitutes of love and companionship that he really missed so much.
Chapter 2 tells us as much when it asks the question of what the guineas really meant to Silas.
It was pleasant to him to feel them in his palm, and look at their bright faces, which were all his own: it was another element of life, like the weaving and the satisfaction of hunger, subsisting quite aloof from the life of belief and love from which he had been cut off.
This is the evidence that money and gold were essentially substitutes of joy in the new life of Silas, one which was lonely and lacked the velocity of his life in Lantern Yard.
For twenty years, mysterious money had stood to him as the symbol of earthly good, and the immediate object of toil. He had seemed to love it little in the years when every penny had its purpose for him; for he loved the purpose then.
Hence, the former "purpose" that propelled Silas to live day by day is no longer there, and he needs a way to make up for the emptiness he feels in his life.
Finally, notice how money and gold also seem to move Silas away from his gloomy depression—or at least that is what he thinks. In a metaphorical way, Eliot explains that the money and gold shine in the darkness. This is an allusion to the state of mind of Silas, and of the false sense of joy that these material possessions bring him.
Silas walked homeward across the fields in the twilight, he drew out the money and thought it was brighter in the gathering gloom.