One of the many problems with making really, really big creatures is that their bones are, eventually, incapable of supporting their own weight. Eventually, they get to be all leg and no body. This is especially a problem when trying to make something that's humanoid but larger than normal, because you have to make the legs elephantine in order for them to support the weight of everything else. The square-cube law is generally a pain in the rear when doing such things.
Now, the obvious solution to this is to make better bones/supportive structures - ones that can withstand more compressive force per unit of bone mass, meaning that you can use less of them, meaning that you don't need to make disproportionately-wide limbs to counteract a creature's mass.
I considered using the goethite nanofiber/protein mixture that limpet teeth use, which has a tensile strength of 3-6.5 gigapascals, but I don't think that that tensile strength translates into the compressive strength required to make an effective bone, since tensile strength is a measure of how durable a substance is when being pulled on, whereas compressive strength is a measure of how durable a substance is when being squeezed.
So: what are some biological substances stronger than bone?
Quality answers to this question will cite a substance that falls into the following four categories:
Either capable of being processed by Earthly biology, or capable of being built out of metabolizable sub-components of itself by Earthly biology. No noble gases or non-reactive substances, here, please, although I wonder how they'd ever be a suitable answer to this. Note that those limpet teeth I previously mentioned are made of such a metabolizable substance, in that the limpet turns iron into ferrihydrite, which is then nucleated and turned into goethite crystals (i.e. those nanofibers I mentioned earlier).
Stronger than bone in terms of compressive strength. Bone has a compressive strength of 170 megapascals. I want more. I can worry about shear loads later, or counteract them by wrapping the compressive core of the bone (i.e. what carries the weight) in biologically-grown limpet-tooth spaced armor with bone marrow in-between as a shock absorber.
Has to be possible on Earth. No frozen hydrocarbons. Especially no neutronium. No, not even a little. Bad Stack Exchange. Bad. Drop. Drop the neutronium. Good boy.
Note that I am not interested in determining what evolutionary pressures might lead to the adoption of such a material as a supportive structure; we're talking mad science and the limits of what's possible here, not boring old evolution. I am also not interested in how this might affect the biology of the creature it's attached to - i.e. things like blood flow issues or increased nutritional needs. All I am interested in is whether there are biomaterials with better compressive strength than bone.
Tagged "science-based" because I want this to be an actual material, and not handwavium, but not "hard-science", because whatever this is has probably never actually been used in a supportive structure in real life.