Yes, you can save it. In my opinion, you will not be able to keep the colored part alive for very long- it doesn't contain any chlorophyll. I would cut the flesh from the rot up, until you hit fresh, untainted tissue. Let the cut cactus rest for a few days in a dry, cool place devoid of high sun and humidity. It will then callous over. You should then place the cactus (the right way up!) in a dry soil mix. It should sprout new roots within a couple of weeks.

~palmsandsnow

(Just wondering about the name - was it sold as ruby ball?)

It is grafted cactus, bottom is different plant than top. The bottom plant is quite hardy and fast growing, if it is rotting, it must have been overwatered quite a bit. It actually looks very wet right now.

It could, possibly, be saved, but you need to resolve problem with overwatering (mix and the pot).

Is there a drainage hole in the pot? What is under the gravel (what kind of 'soil'?)

A few other things you will need:

• Make sure the pot you put it in has a drainage hole- very important.
• Get some perlite and cactus & succulent mix, and do a 60% perlite, 40% C&S combo. That should prevent rot in the future.
• When you repot, you should wait a few days before you water. After that, water every 1 and 1/2 to 2 weeks. Adjust your schedule based on how your plant reacts.

I don't think it had a label unfortunately. I've had to guess what it might be from some research. The soil is compost with gravel mixed in and there is a layer of gravel at the bottom for drainage. I've had it for a few weeks and only give it about a egg-cup full of water every 12 days.

The gravel layer at the bottom doesn't help, it actually makes things worse. You need to get the plant out asap, before the whole thing rots.

Succulents need to be watered thoroughly, not in sips. In well draining mix, all extra water will drain out and shouldn't sit at the bottom of the pot. Succulents need to be watered when the mix is just about completely dry, all the way - not only top inch or two.

You need to cut rotting part off as palms already described, pls. read both posts for good instructions.