The Sixth Foundation Stone: The Sard
"The Sard, which is entirely of a blood-red colour, signifies the glory of martyrs, of which it is said,“precious in the sight of the Lord is the death of His saints,” and is with reason put in the sixth place,seeing that our Lord was incarnate in the sixth period of the age, and was crucified on the sixth day of the week for the salvation of the whole world."
[The Explanation Of The Apocalypse, by St. Bede the Venerable, 7th Century Doctor Universalis]