I am not a mason, or an expert on fireplaces. (It is not a bad idea to bring in several such people to get an estimate on the repair.) However, a few things seem fairly clear.
The crack appeared because...
- The hearth may have been insufficiently reinforced, thus too little rebar, metal mesh, or nothing at all.
- Heat stresses, caused by heating and cooling cycles can cause cracks. A properly built hearth should have been designed to prevent that.
- Insufficient foundation under the hearth, which might have allowed it to settle, thus introducing stresses.
One or more of the above factors will have caused the problem. Someone experienced in seeing these types of problems will know what caused it, and thus what is needed to repair the problem so it does not happen again.
For example, it should be obvious if there is no reinforcement at all, since the crack will let you see inside. If there was settling, then one of the pieces may now be tilted relative to the other. There may also be other cracks in the walls or foundation of the house to be found.
If you do nothing, then there are some issues to consider beyond pure aesthetics. The one that comes to mind immediately is the crack will allow infiltration of air into the house from the ground. If there is radon in that air, it could potentially cause lung cancer over a long period of time. While this is not a high probability event, it is possible, and it does happen.
Repair may be a moderately difficult thing, since you really need to eliminate the underlying problem. Simply slapping some mortar on the crack will not solve a thing, even if you bother to find some heat resistant mortar. The crack will just reappear.
Again, I'd suggest contacting a good mason who understands fireplaces and foundations.
Yes, you can paint it.
Personally, I always use oil-based paints because they last longer, but you can use whatever paint you want (water colors?)
Concrete is porous so you might want to use a grain filler or sealer of some kind before painting.
There are thick paints specifically designed for painting concrete, but they tend to look pretty industrial.
Best Answer
Quikrete FASTSET Repair Mortar is a great product. The biggest difference between it and a typical type S mortar is that it is "fastset" (duh) meaning it will be hard in about 20 min. Not cured, but hardened. And that it is intended for commercial use as it will eventually reach 6000 psi after it's full 28 day cure. This product does not need any bonding agent as it is already added. http://www.quikrete.com/ProductLines/FastSetRepairMortar.asp
Any regular type S mortar will harden in about a day or two depending on the enviorment. And will eventually reach 1800psi http://www.quikrete.com/ProductLines/MasonMix.asp
You may think "oooo stronger is better" but not so with mortar. You always want the mortar to be weaker then the surrounding material (stone, brick, cinder block) That is why it comes in so many strengths. From strongest to weakest type, M S N O K.
Sorry to be so long winded.
Basically, go with the type S, and use the FastSet Repair mortar for repairs around the house.