The solution commonly used for the heaviest stones is a mixture of nitrate of thallium and nitrate of silver.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin
Sir William Crookes, of course, is best known as a great chemist, discoverer of the element thallium, and inventor of numerous scientific instruments; while Sir Oliver Lodge's most striking work has been in electricity, and more particularly in the direction of improving wireless telegraphy.
"Historic Ghosts and Ghost Hunters"
H. Addington Bruce