Refining bauxite into aluminum is an extremely involved process, and corundum is aluminum oxide.
So, if your planet was rich in aluminum but poor in iron and other heavy metals (at least in the crust, you definitely want iron in the core), then the inhabitants might find lots of rubies and sapphires lying around, but without any "easy" ores to refine into tin, copper, or iron, they might never develop metallurgy needed to advance out of the Neolithic.
A big problem is that the red color of rubies comes from chromium, and the blue of sapphires comes from iron and titanium.
Another problem is that the main igneous source of corundum is nepheline syenite which is relatively rare since it comes from a silica-poor magma. Magma containing a lot of silica gobbles up the aluminum and produces feldspar instead of nepheline. It's very, very difficult to imagine an Earth-like planet with a silica-poor crust! Of course, an incrementally larger amount of aluminum might produce enough more nepheline for your purposes.
Not only that, on Earth, crustal iron played a big role in the evolution of Earth's atmosphere. As cyanobacteria produced oxygen in the Archaean and Proterozoic, the oxygen combined with iron dissolved in the water and settled to the bottom, producing the banded iron formations that have been a major source of iron ore throughout history. Without the iron, the atmosphere might have too much oxygen!
Finally, vertebrates on Earth need iron in hemoglobin molecules in carry oxygen to their tissues. Other creatures use other pigments in their blood, based on copper. I don't know if there would be a suitable aluminum or magnesium based pigment.
Finally, the first iron worked by humans was "skymetal"; that is, it came from meteorites. It's difficult to imagine a planetary system containing an Earth-like planet without iron meteorites. But they were rare and iron objects made from them might be just enough iron for your world.