It is doubly refractive, whereas spinel and the diamond, which two it closely resembles, are singly refractive.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin
Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc.
"The Economic Aspect of Geology"
C. K. Leith
Emery is an intimate mechanical mixture of corundum, magnetite, and sometimes spinel.
"The Economic Aspect of Geology"
C. K. Leith