  We discuss the nonlinear elastic response in scale invariant solids.
Following previous work, we split the analysis into two basic options:
according to whether scale invariance (SI) is a manifest or a spontaneously
broken symmetry. In the latter case, one can employ effective field theory
methods, whereas in the former we use holographic methods. We focus on a simple
class of holographic models that exhibit elastic behaviour, and obtain their
nonlinear stress-strain curves as well as an estimate of the elasticity bounds
- the maximum possible deformation in the elastic (reversible) regime. The
bounds differ substantially in the manifest or spontaneously broken SI cases,
even when the same stress-strain curve is assumed in both cases. Additionally,
the hyper-elastic subset of models (that allow for large deformations) is found
to have stress-strain curves akin to natural rubber. The holographic instances
in this category, which we dub black rubber, display richer stress-strain
curves - with two different power-law regimes at different magnitudes of the
strain.
