ΔE 26.3, loose: different enough that you should pick on purpose. Sapphire Gem is noticeably lighter, much more saturated, cooler than Necromancer Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Sapphire Gem is noticeably lighter, much more saturated, cooler than Necromancer Purple.
The same difference applies the other way round: Necromancer Purple is noticeably darker, much more muted, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Necromancer Purple on the colour wheel See Sapphire Gem on the colour wheel
All matches: Necromancer Purple equivalents · Sapphire Gem equivalents