Adjustable Knob Solid and well-built, this Round Granite Umbrella Base made from granite stone, will be the best choice for anyone looking for a heavy duty, durable weather resistant base to anchor your patio umbrella. The Granite Umbrella Base includes interchangeable sizing inserts and adjustable tightening knob that provides support to most umbrellas. It is also sturdy enough to be used as a standalone piece. This umbrella base will provide your umbrella and you support for many years to come along with many years of satisfaction