The Boring Company finds something that is profitable to do with all that waste rock from digging tunnels.
Elon Musk is not a man to miss an opportunity, and he's just discovered a profitable way to deal with all the waste rock. The Boring Company has piling up: turn it into Lego!  The growing pile of rock is what is leftover when using the underground sleds. Rather than paying to have the rock disposed of, Musk will create a “lifesize” Lego-like interlocking bricks “that can then be used to create sculptures or buildings.
In Musk's tweet announcing the new Boring Company product, he points out the rock is “valued for California seismic loads” meaning it is very strong. However, it's hollow in the middle and therefore not very heavy.
New Boring Company merch coming soon. Lifesize LEGO-like interlocking bricks made from tunneling rock that you can use to create sculptures & buildings. Rated for California seismic loads, so super strong, but bored in the middle, like an aircraft wing spar, so not heavy.– Elon Musk (@elonmusk) March 26, 2018
In a later tweet, Musk also confirms kits will be offered with the first being for ancient Egypt. It will include interlocking bricks that allow you to build pyramids, a Sphinx, the Temple of Horus, and some other well-known landmarks. Unfortunately, pricing has yet to be confirmed, but I imagine shipping costs will be quite high and international delivery rates extortionate.
Lego-like bricks is the third side-product The Boring Company is going to offer. We've already had branded hats and a flamethrower. We have to assume that the brick kits will be flamethrower proof, and someone is sure to test that and post the evidence on YouTube.
Once SpaceX makes it to Mars and sets up a transportation link back to Earth, I think we should expect the brick range to expand with very limited editions. Mars does have a lot of rock, after all.