"High in calcium," "low in sodium," "potassium-rich," "lacking copper balance": the elements are named in their free state, not in their actual combinations with each other. It is necessary to learn a little basic physiology to appreciate this method of categorizing plant contents. All the chemistry of foods, fluids, etc. ingested daily goes through a rearrangement in various parts of the digestive apparatus and with associated organs like the liver and pancreas. What goes in as a food or plant rich in certain compounds will emerge for distribution through the blood after a game of chemical musical chairs.
Therefore, when an herb is labeled "rich in calcium," there will be a predominance of calcium compounds available for breakdown by the liver, for example, into other calcium compounds. During this activity, the free ions of calcium cross the floor to dance with another chemical partner. The whole process introduces a calcium-rich environment as this changeover is taking place. Any calcium compounds needed by the body will be eagerly combined.
So an herb rich in calcium will be expected to provide nutrients for bones, teeth, nails, hair, and nerve fibers if the digestive tract can assimilate them out of the food. When you read of a plant which improves the health and function of bones, teeth, nails, and so on, you would expect to find it rich in calcium, the mineral most needed for these body parts to function well.