What was the curios aspect of the ring on the body of Mr.Douglas?

Douglas always wore the ring with the rough nugget above his wedding ring.

“They’ve taken his wedding ring!” he gasped.

“What!”

“Yes, indeed. Master always wore his plain gold wedding ring on the little finger of his left hand. That ring with the rough nugget on it was above it, and the twisted snake ring on the third finger. There’s the nugget and there’s the snake, but the wedding ring is gone.”

“He’s right,” said Barker.

“Do you tell me,” said the sergeant, “that the wedding ring was below the other?”

“Always!”

“Then the murderer, or whoever it was, first took off this ring you call the nugget ring, then the wedding ring, and afterwards put the nugget ring back again.”

“That is so!”