I write a small description of the specimen at the bottom, such as:
- Gray metallic massive
- Pencil-shaped crystals
- Green, pink, and blue Xtals on matrix [Xtals = crystals]
- Blue XOM [XOM = crystals on matrix]
- Blue mXOM [mXOM = microscopic crystals on matrix]
- Massive; from type locality
- Fragile! Don't touch Xtals!
- Water soluble white XOM
- Poisonous gray octahedra OM
- Radioactive yellow powdery OM
- et cetera.
That is, the description should be a short version of anything you would say to a beginning collector who wanted to look at the specimen.
Also, you may want a border, signature, or "from the collection of..." statement on each of your labels.
I put indexing information on the back of my labels. I am a systematic collector, so the label gets the Dana numbers (from Dana's System of Mineralogy, currently in its eighth edition) for each of the minerals present.