Take a chunk, place it in a glass bowl. Pour some vinegar over it so that it's submerged in the vinegar. Let soak for 24-hours. Look at it. IF the solids are either gone, or diminished it is being dissolved by the acid in the vinegar and almost certainly calcium based. Heating the water creates deposits on the inside of your WH unless the water is soft, similar to what happens in your teakettle.
SOmetimes, you can flush some of it out, but it depends on how bad and how much is there...it might become too big of a lump to get out.
Typically, the lining doesn't crack off...it gets pinhole leaks and then rusting starts on the tank. If you get rusty water out, your tank's life is almost up. A good anode can help slow the process. Excessive mineral deposits can create some hot spots that shorten the life somewhat. It also slows the response time to reheating the water and in the extreme case, limit how much hot water is available since so much of the tank gets full of those deposits.
