31t Ninety-Third Psalm -- what is 'the topmost stone'

Grace all the work shall crown

Through everlasting days;

It lays in heav’n the topmost stone,

And well deserves our praise.

What is "the topmost stone"?  I don't know. I'm not quickly thinking of any specific Bible verses, and a search of the KJV doesn't find 'topmost' or 'top most' (and I haven't scanned the 417 matches to 'stone.')

So, just thinking about it, it makes one think of either a keystone in an arch -- the topmost stone that allows the arch to stand and be strong -- or just the capstone or highest bit of a building. Note that 'grace' is not itself the topmost stone, but rather it is grace that places the topmost stone in heaven. The fulfillment of heaven is the work of grace. So, anyone want to point out a reference or meaning that I'm missing?

When singing this song, I invite you to ponder that grace is always scandalous -- it is defined by people getting some good they don't deserve.

Philip Doddridge was a non-conformist minister, a contemporary of Isaac Watts. Ten tunes in the 1991 book have words by him. His 400 some hymns were mostly written to put the content of his sermons into singable form, to immediately reinforce the message he was bringing to his congregants--a wise teacher.

I'm not seeing any particular connection between these words and the 93rd Psalm of the tune name (the words precede the tune), but here's the Psalm if anyone wants to look for themselves:

Psalm 93

¹The Lord is king, he is robed in majesty; the Lord is robed, he is girded with strength. He has established the world; it shall never be moved;

²your throne is established from of old; you are from everlasting.

³The floods have lifted up, O Lord, the floods have lifted up their voice; the floods lift up their roaring.

⁴More majestic than the thunders of mighty waters, more majestic than the waves of the sea, majestic on high is the Lord!

⁵Your decrees are very sure; holiness befits your house, O Lord, forevermore.