Crossword puzzle clue "the 2x2 black square near the middle of this puzzle's grid e g which is part of eight answers"

The 2x2 black square near the middle of this puzzle's grid, e.g., which is part of eight answers

Last letter in most plurals (but not in this puzzle's six longest answers, which are the only plurals in this grid)

Number of times this puzzle's theme word appears diagonally in the grid (to complete the puzzle, shade those words plus the Down answers that begin where they meet)

Following the nine black squares in the center of the grid, a hint to five pairs of answers in this puzzle

Gritty ... and a description of this puzzle, which is also a hint to completing eight answers

Builder of a 2x2 vessel?

Brief version of this puzzle's title hidden in eight long puzzle answers

Diamond deception found in this grid nine times: eight in square four-letter clusters, the ninth formed by the clusters' outline