Rocks and minerals all have different characteristics, including hardness and lustre. The Mohs Hardness Scale is the primary scale that tests a rock's hardness by how easily it can be scratched. Diamond has a score of 10 on the hardness scale, which is the highest a mineral can obtain. Talc has a score of 1, and gypsum has a score of 2, which makes these two minerals similar and difficult to differentiate between.
Feel both pieces of rock for how slippery they are. If the rock is slippery, it may be talc. If the rock isn't slippery it may be gypsum.
Fragment pieces of each rock with your fingernail. If cleavage fragments fall off and are tiny, the specimen is talc. Cleavage on talc is perfect. Cleavage is the quality of a split along defined planes of a rock or mineral. In this case, you would be splitting the rock with your fingernail.
Scratch each piece of rock. If powder falls off and doesn't feel slippery or greasy, it's gypsum.
Check the colours of each. Talc will have a grey, white, green or silver colour to it. It can also have a dull, pearly or greasy lustre. Gypsum can be colourless, white and grey with yellow, red and brown shades to it. Gypsum can also have a pearly lustre to it.