Yeah looks like Mica and sandstone. Unfortunately landscapers/contractors love to just cover things up with top soil and run away.
The rocks could be tightly packed silt, loose sandstone, or maybe mica from excavation. Pictures would help.
The key with windows is reducing your cost. I too received a quote for $30-40k back in the day... so I did it myself with vinyl replacements (double pane, argon, etc). Ended up spending only a few thousand total for the whole house, I know I did a better job than the local contractor because I spent my time and did it right.
We had a massive comfort difference due to the old windows being extremely drafty (1960's original wood single-pane). In my opinion that's the biggest benefit. We also were able to remove our storm windows, so improved exterior visibility, reduced cleaning, and improved curb appeal in my opinion.
Energy-savings, yeah not so much unless you DIY like I did to keep the cost down.
