Crystal form matters more than size
A small cabinet specimen with sharp euhedral crystals, clear color, and a clean matrix is worth more than a larger piece with chipped corners and dull faces. Look at the terminations — are the crystal points complete and undamaged? Are the faces flat and reflective rather than etched and dull? Is the matrix natural rock or has it been trimmed/glued?
What to look for
Sharp crystal edges. Undamaged terminations. Saturation of color (intense vs. washed-out). A matrix that supports the crystals naturally rather than 'just a base.' No glue, no repair, no dye, no heat treatment — and if any of these have been done, full disclosure from the dealer. Provenance: which mine? When collected? Who collected? Reputable dealers welcome the questions.
Starter species
Species that consistently produce affordable, beautiful cabinet specimens for first-time buyers: fluorite (especially from Yaogangxian and Shangbao), calcite from Daye, golden barite from Jiangxi, Chinese azurite, pyrite cubes from Spain or Hubei. Avoid first-time purchases of stibnite (fragile), vivianite (fades), realgar (fades), and similar fugitive species until you've built care discipline.