EFFICERE 22-Ounce All Steel Rock Pick Hammer with Pointed Tip, 11-Inch Overall Length | Essential for Geological Study, Rock Hounding, Prospecting, Mining, Fossil Dig, Masonry Related and Much More

Go to Amazon, type in geology hammer. Sort by cheapest. Get cheepst that will arrive in time for your field trip. If you get fancy look for one with a few reviews. Like this one, 4.6 stars for 14$ will do nicely. Just don't hammer basalt without eye protection. https://www.amazon.com/Choice-22-Ounce-Pointed-11-Inch-Overall/dp/B074GYGQXY/ref=mp_s_a_1_5