The most frequently used laser-active rare earth ions and host media together with typical emission wavelengths:

neodymium (Nd3+)
YAG, YALO, YVO, YLF, KGd(WO4)2, KY(WO4)2
1064, 1047, 1053, 1342, 946 nm

ytterbium (Yb3+)
YAG, tungstates (e.g. KGW, KYW, KLuW), YVO4, borates (BOYS, GdCOB), apatites (SYS), sesquioxides (Y2O3, Sc2O3)
1030, 1020–1070 nm

erbium (Er3+)
YAG, YLF
2.9, 1.6 μm

thulium (Tm3+)
YAG
1.9–2.1 μm

holmium (Ho3+)
YAG
2.1, 2.94 μm

cerium (Ce3+)
YLF, LiCAF, LiLuF, LiSAF, and similar fluorides
0.28–0.33 μm

Common transition metal ions in laser-active crystals:

titanium (Ti3+)
sapphire
650–1100 nm

chromium (II) (Cr2+)
zinc chalcogenides such as ZnS, ZnSe, and ZnSxSe1-x
2–3.4 μm

chromium (III) (Cr3+)
Al2O3 (ruby), LiSrAlF6 (LiSAF), LiCaAlF6 (LiCAF), LiSrGaF6 (LiSGAF)
0.8–0.9 μm

chromium (IV) (Cr4+)
YAG, MgSiO4 (forsterite)
1.35–1.65 μm (YAG), 1.1–1.37 μm (forsterite)

These are only the most common host crystals; many others exist, which are less frequently used.