Highlands vs. Lowlands Geologies
Like on all planetary crusts and the dirt under your feet here on Earth, lunar material consists predominantly of silicate minerals, i.e., silicon and oxygen molecularly bonded to various metal atoms. However, the lunar highlands differ from the lunar lowlands mainly in their concentrations of the metal oxides.
The early lunar crust formed a "magma ocean" which solidified into a crust of the lightest minerals which had floated to the top, predominantly aluminum calcium silicates ("anorthositic material") about 4.5 billion years ago. In fact, this crust is quite rich in aluminum and calcium compared to Earth's crust. (Earth's crust is split into two layers, the top being richer in aluminum silicates, "SiAl", with an underlying layer of magnesium rich silicates, "SiMg".)
However, the period between 4.5 and 4.0 billion years ago was marked by heavy bombardment by meteors and asteroids, causing intense cratering.
The highlands geology is mostly composed of overlapping layers of material ejected from craters, predominantly the initial anorthosite (aluminum rich) crust. Rocks brought back from the highlands vary in age between 3.84 and 4.48 billion years old.
Today's flat lowlands "mare" regions ("mare" is Latin for "sea") formed about 4 billion years ago when immense asteroid impacts fractured the crust, allowing the lavas from 300 kilometers deep (200 miles) to erupt through the impact fractures and form vast seas of lava. (For comparison, Earth's crust today is 50 kilometers, or 30 miles, thick in most places.) This material was poor in aluminum and calcium, but rich in iron and magnesium. However, the lavas melted preexisting aluminum-rich surface materials and mixed with them. The lava was rich in the heavier radioactive elements which had initially settled well below the crust, and the radioactivity kept the molten seas of lava hot for millions upon millions of years. Mare rocks have been measured to be between 3.15 and 3.77 years old. The last molten lakes are thought to have finally solidified about 3 billion years ago.
The resulting material which makes up the surface of these ancient lava seas is rich in iron and magnesium minerals, with a remarkably high content of titanium minerals.
When you look at the moon, the mares are the darker areas, and tend to be circular in shape because they formed in giant asteroid impact spots.