A ruby is just a red sapphire. Both are the mineral corundum, a type of aluminum oxide, rubies just have a chromium impurity making them red.
Rubies and Sapphires are sold as separate stones, but are actually the same mineral! That mineral would be Corundum a crystalline form of aluminium oxide with traces of iron, titanium and chromium. Depending on the color, the mineral is called by different names.
Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire. Green corundum, for example, is called a "green sapphire." The different colors are caused by different impurities in the mineral.
Rubies are rarer than sapphires and so cost significantly more. As with all gemstones, the price depends on the size and quality of the stone, but rubies will almost always have more value.
The price also depends on the extent to which the gem is treated. To find a stone without flaws is very rare, and so these are sold as "untreated" stones for a lot more than treated gems.
Green, yellow, pink, white sapphires all rarer, and thus more expensive, than normal, blue sapphires. Medium rich to light blue sapphires have more value than the darker more common sapphires.