ΔE 34.7, loose: different enough that you should pick on purpose. Ignis Fatuus is noticeably lighter, much more muted, warmer than Sapphire Gem.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). Ignis Fatuus is noticeably lighter, much more muted, warmer than Sapphire Gem.
The same difference applies the other way round: Sapphire Gem is noticeably darker, much more saturated, cooler.
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 Sapphire Gem on the colour wheel See Ignis Fatuus on the colour wheel
All matches: Sapphire Gem equivalents · Ignis Fatuus equivalents