J. S. Fletcher 
Orange-Yellow Diamond 

When an elderly pawnbroker is murdered in the London parish of Paddington, a young, down on his luck writer is accused of the crime. But then it’s found the pawnbroker had had in his possession an extraordinary South African diamond worth over eighty-thousand pounds — a diamond that’s now missing. It falls to Melky Rubenstein to unravel the mystery and prove the young man’s innocence. But what is the significance of the Spanish manuscript? What part do the Chinese medical students play? And what about the mysterious Mr. Mori Yada? (Goodreads)