Add magma mod, adds... magma! It can be created by putting lava on cobblestone.

magma/init.lua

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+--- Magma block!
+
+magma_texture = {{
+	name = "magma.png",
+	animation = {
+		type = "vertical_frames",
+		aspect_w = 16,
+		aspect_h = 16,
+		length = 8,
+	},
+	backface_culling = false,
+}}
+
+minetest.register_node("magma:magma", {
+	description = "Magma",
+	tiles = magma_texture,
+	special_tiles = magma_texture,
+	is_ground_content = false,
+	damage_per_second = 2,
+	light_source = 8,
+	groups = { cracky = 3 },
+})
+
+minetest.register_abm({
+	label = "Generate magma",
+	nodenames = {"minecraft:cobble"},
+	neighbors = {"minecraft:lava_source", "minecraft:lava_flowing"},
+	interval = 1.0,
+	chance = 1,
+	action = function(pos, node, active_object_count, active_object_count_wider)
+		minetest.set_node(pos, { name = "magma:magma" })
+	end
+})

magma/mod.conf

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+name = magma