The line "So Eden sank to grief" contains allusion or in other words literary reference to the Biblical anecdote of Adam and Eve who were in the Paradise until they ate the fruit of  The Forbidden Tree". Read: Summary of Nothing Gold Can Stay Analysis of Nothing Gold Can Stay

"Nothing Gold Can Stay" by Robert Frost is a short poem having just 8 lines. The poem is about nature like most of the other poems of the poet. Read: Summary of Nothing Gold Can Stay Analysis of Nothing Gold Can Stay

It is an example of Allusion which is a literary device. It means a passing reference or mention of something from history or anywhere else. Here, it refers to the biblical Garden of Eden. Read: Summary of Nothing Gold Can Stay Analysis of Nothing Gold Can Stay

The tone of the poem is fatalistic. The poet is in despair that Nature itself reveals this to us that nothing is permanent and everything is transient. Read: Summary of Nothing Gold Can Stay Analysis of Nothing Gold Can Stay

Realistically, it describes the earliest phase when the leaf comes out of a plant. For a very short period, its colour is gold and very soon it becomes green. So for this short period, it looks more like flower. Read: Summary of Nothing Gold Can Stay Analysis of Nothing Gold Can Stay