Number 56 (Story #3), November 15, 1991 by Phillip F. Schewe and Ben Stein CARBON-13 DIAMONDS have the largest atomic density of any known solid. A General Electric/Ford collaboration led by William F. Bannholzer and Thomas R. Anthony at GE has studied the properties of synthetic diamonds with differing isotopic compositions, ranging from nearly-pure carbon-12 to nearly-pure carbon-13 (H. Holloway et al., Physical Review B, 1 October 1991). They found that the density of the C-13 diamonds was larger than that for natural diamonds by a factor of 0.04%, a smaller variation than had been expected from theoretical estimates. Practical applications of C-13 diamonds are unlikely unless the synthesis procedure, involving high pressures and temperatures, can be simplified. (Science, 1 Nov. 1991.)