I have a jade plant with tubular shaped leaves; I bought it 2 years ago. It is in a large pot placed next to a large window with Western exposure; it is "mulched" with polished small stones. I've been watering it every 3 weeks, giving it 6 cups of water. It did fine until a few months ago, when it started dropping leaves, not just dried and small ones, but large green ones. I noticed this past week that an inner stalk was blackened from the base half way up, now it is fully black. A gardening guru suggested to me that it has a fungus and the best thing to do is get rid of it. Any other thoughts?

A fungus is unlikely. Root damage is more likely. Please, post a photo that shows the entire plant, including its pot.

Yes, a photo will help to determine the issue and whether your plant can be saved. It definitely sounds like a root issue, most likely root/stem rot from too much water.

I have posted three photos. The first (plant) is of the whole plant in its pot next to its "western" light window. The second (typical) is a close-up of typical dried leaves, which have shedded like this since purchase two years ago. The third (black) shows the blackened stalk at the center of the plant, which appeared within the past 2 weeks; at the base is a "nest" of dried leaves. I do appreciate any help you can offer.

Overall, your Crassula looks healthy and beautiful! A limited amount of older leaf loss is normal as long as it continues to get healthy new growth.

If the soil gets a bit too dry between waterings, that may cause more leaf loss than normal. Can you feel the soil between the stones and dead leaf tissue on the soil surface? Does the pot have drain holes?

Without knowing more, it seems that you may be waiting too long between waterings (3 weeks) and may then be providing too much water (6 cups). It might be better if you were to use your finger or a chopstick as a probe to determine how dry the soil was getting between waterings. I also worry about water accumulating in the bottom of the pot if it does not have drain holes.

Consider removing the stones and dead plant tissue from the soil surface as they serve no useful purpose.