I don't feel like I should have to explain what a rock is to you.
Rock price in Growtopia: sells for about 1.5 WL, buyers pay about 1 BGL (median of 169 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: BBNFN, HOH3, OLIV, LOVEWILLOW, ARROW