We are developing very-high resolution detector blocks for animal PET applications. We studied different scintillating crystals materials, BGO, GSO, LSO, MLS, LGSO, and LYSO, to determine the most suitable material for a low cost and high-resolution detector. In this study, we measured and evaluated two different light-output data from individual crystal samples (two different sizes of needles): The scintillation light output received by the PMT as used in PET (LOPET) and'the intrinsic light output (ILO). The ILO data, were measured with the largest side of crystal needles coupled to the PMT (crystal needles "lying down onto the PMT"), the LOPET data, were measured with the smallest crystal end coupled to the PMT as used in a PET detector (the crystal is "standing up" on the PMT). The pulse-height spectra for both ILO and LOPET were acquired for all the individual crystal samples, for deducing the percentages of light loss from self-absorption (LLSA) in the PET detector configuration, and energy resolution in both positions (ILO-ER and LOPET-ER). With these crystals, four detectors blocks were also developed, and we measured the overall light output and the position-decoding maps to gauge the decoding capability (DC) of the crystals. We also compared the visual color and clarity of individual crystals and detector blocks. For the two different sizes of the crystal samples that we received, we have the following finding: (a) For the 1.3×1.3×10 mm crystals "standing up" as used in a PET camera, GSO, MLS and LYSO, lost almost half of the light while LSO lost more than 2/3 thereby ended up with the same light output as GSO. The energy resolution of LSO and GSO are quite similar (16% and 15%); but the light absorption is significant higher in LSO (71%) in comparison to GSO (54%), MLS (52%) and LYSO (48%). (b) For the 2×2×10 mm, the light loss from self-absorption (LLSA) was less than the 1.3 mm samples. MLS has the lowest self-absorption (26%), while GSO and LSO have the highest light loss. LGSO and LSO have the lowest LOPET for all the lutetium crystals tested. (c) From the position-decoding results from the detector blocks, LYSO, MLS and GSO provided better position-decoding resolution than the LSO. (d) For the visually comparison, LSO were visually much darker than the other Lutetium crystals.