Hi,

Concrete, adobe, ... most things that are dense.
Dark colors absorb better, but middling colors do pretty well also.

Wood floors will store some heat, but not as much as the materials above. Carpet is probably the worst.

Solar energy that comes through a window will nearly always end up getting absorbed somewhere in the room. If it gets reflected off the floor that it hits first, then it will likely get absorbed by the wall or ceiling -- so you don't lose the heat.
The advantage of having a dark and thermally massive floor is that the floor will absorb a lot of energy and then give it back later in the day after sunset. If you have a lot of south facing glazing, then this not only stores heat for later, but also prevents overheating during the sunny part of day. But, if you don't have so much south facing glazing, then a normal room without a lot of added thermal mass still has enough thermal mass not to overheat. Most of the rough rules of thumb say that you can go up to about 6% of your floor area in south facing glazing without worrying too much about adding thermal mass.

One of the most important things to think about in passive solar heating is that windows gain a lot of heat during the day, but also lose a lot at night. You can greatly improve performance by providing some form of night insulation for the windows (thermal shades, ...).

You might want to try looking through some of the design guides at the link I attached. The Chiras book thats listed there is (I think) also good place to start.

Gary

Here is a link that might be useful: Passive solar home design guides

Thank you for your reply. We are considering buying an earth berm home with passive solar heating that is for sale in Wyoming. It was built in 1983 and has all original decor, so it needs some updates. We're trying to research what our options are for materials that won't negatively impact energy efficiency.