Six items identify almost every common rock and mineral. Everything else is a convenience. Here is what each one is for, and what it costs you in weight.
Hardness is the fastest test and the knife is the hardest tool you will normally carry. A steel blade is about 5.5 on Mohs' scale, which splits the common minerals into two groups: those a knife cuts (calcite, halite, galena, gypsum, talc) and those it cannot touch (quartz, feldspar, pyrite, garnet). Do not use the blade of a good knife for scratching rock; carry a cheap one, or use the spine.
Ten power is the standard because it resolves the things that matter without requiring you to hold the specimen still for long: the shape of sand grains, whether feldspar is fresh or altered to sericite, whether a green mineral is chlorite or epidote, and whether a dark rock is fine-grained or glassy. A triplet lens gives a flatter field than a cheap doublet, but a doublet at 10x is genuinely adequate. Keep it on a cord around your neck, because a lens in a pocket is a lens you will not use.
The unglazed back of a bathroom tile works, costs nothing, and will identify hematite, magnetite, pyrite and gold in seconds. The plate needs to be harder than the mineral being tested, which is why unglazed porcelain (about 7) works for everything except quartz and its relatives. Carry a small piece with fresh white surface and replace it when it becomes a mess of coloured lines.
A 10 per cent solution separates the carbonates from everything else in one drop. Calcite fizzes violently in cold dilute acid; dolomite only fizzes when powdered; limestone fizzes on a fresh surface; marble fizzes and is crystalline. Carry it in a small plastic dropper bottle with a tight cap, keep it away from your notebook and your camera, and treat it as a chemical, because it is one. Never put acid in an unlabelled container and never use a bottle that could be mistaken for drinking water.
A small neodymium magnet on a cord identifies magnetite instantly and separates it from hematite, which is the pair most often confused in dark heavy specimens. It also detects the magnetite that concentrates in black sand, and it will tell you whether a specimen of unknown black metallic mineral is worth testing further. A fridge magnet is too weak; buy a 10 mm neodymium disc.
The most underrated item in the kit. Record the location, the host rock, the orientation of the outcrop, and what you actually observed rather than what you concluded. Write the hardness number, not the word hard; write which test you ran, not just the answer. A pencil works when it is wet and does not run when the page gets soaked, which a pen does. This is the item that turns a walk into a record.
A geological hammer is useful for extracting fresh material but is also how most field collectors do damage they did not intend — to protected sites, to outcrops others rely on, and to their own specimens, which shatter when struck badly. Do not take a hammer to any site where collecting is restricted, do not hammer a face that other people use for teaching, and remember that a specimen you cannot carry out intact is better left in place with a photograph and a note.