Just spelling out that we have a much better idea now what the first lines of the book mean:

Beneath the moonlight glints a tiny fragment of silver, a fraction of a line...

(black robes, falling)

...blood spills out in litres, and someone screams a word.

The silver likely refers to:

Neither of them noticed the tall stone worn as though from a thousand years of age, upon it a line within a circle within a triangle glowing ever so faintly silver, like the light which had shone from Harry's wand, invisible at that distance beneath the still-bright Sun.