Norbergite, Mg₃(SiO₄)(F,OH)₂, occurs in the Franklin marble as small grains that fluoresce a pale yellow colour in ultraviolet radiation.

Also occurring as small grains in the marble is the mineral diopside, CaMgSi₂O₆. Diopside fluoresces blue

When both norbergite and diopside occur together in Franklin calcite some very attractive fluorescent mineral specimens are obtained.

The picture shows a colourful specimen of Norbergite (fluorescing pale yellow) and Diopside (fluorescing blue) in short wave ultraviolet radiation. The specimen is from Ogdensberg, New Jersey.