Yes, the most easily absorbed type of calcium is calcium citrate.
http://healthletter.mayoclinic.com/editorial/editorial.cfm/i/356/t/Building%20blocks%20of%20bone/Calcium citrate
This type of calcium supplement is the most easily absorbed, is less likely to cause abdominal discomfort and doesn't have to be taken with meals.
There was an interesting study of nurses and kidney stones that showed a decrease in stones among women who had a high calcium intake from diet, while those who took supplements were 20% more likely to develop stones. It was theorized that the timing of when women took the stones contributed to the likelihood of getting a stone. They recommend that calcium supplements only be taken with meals and preferably with meals containing oxalates, as calcium binds with oxalates and then cannot be absorbed and won't pass through the urine. This seems contrary to what we may believe, but the mechanics of it does make sense.
http://www.medicinenet.com/script/main/art.asp?articlekey=1887&page=2The relationship between dietary calcium and kidney stones is complex. It is known-and this study confirms--that increasing amounts of dietary calcium are associated with fewer kidney stones. It is believed that this occurs because the increased amounts of calcium from food binds to oxalate from food in the intestine, and the complex of calcium and oxalate cannot be absorbed into the body. This reduces the amount of calcium and oxalate that needs to be eliminated from the body in the urine and prevents the formation of calcium/oxalate stones in the kidneys. In fact, calcium supplements have been used successfully to decrease the elimination of oxalate in the urine.
Why then, were calcium supplements associated with more kidney stones in the nurses? The study's investigators hypothesize that the reason is the time at which the nurses took the calcium supplements. The calcium and oxalate in food can bind to each other only if they are present at the same time in the intestine. If calcium supplements are taken with meals containing the most oxalate, everything is fine, and the calcium and oxalate bind. If calcium supplements are taken with meals that contain little or no oxalate, the oxalate doesn't bind to calcium, and the oxalate is absorbed from the intestine and then eliminated in the urine. Furthermore, if calcium supplements are not taken with food, more calcium is absorbed and eliminated in urine. (This would increase the risk for developing stones.) It turns out that 57% of the nurses in the study taking supplements were not taking them with meals, and 24% were taking the supplements only with breakfast, the meal least likely to contain oxalate. Thus, more than three-quarters of the nurses taking calcium supplements may have been absorbing and eliminating more calcium in their urine or were not gaining the benefit of the binding of calcium and oxalate in the intestine.
The study's investigators conclude that a high intake of dietary calcium reduces the risk of kidney stones but supplemental calcium increases the risk. They suggest that the risk of kidney stones among women taking supplemental calcium might be reduced by consuming the supplements with meals, particularly meals which contain the most oxalate-lunch and dinner.
So, to decrease the chances of getting stones, take calcium with meals containing oxalates, and work on getting more dietary calcium and less supplemental calcium.
But beyond calcium, which is not the most important part of the equation, what is your vitamin D level? Is the calcitriol doing the job? This has far more to do with bone strength than calcium. Are you also taking daily zinc, which is proven to strengthen the bones of thals? And are you getting adequate weight bearing exercise to strengthen your bones? And, you should also understand that calcium should not be taken alone. Not only should it be taken with vitamin D, but it should be taken in a bout a 2:1 ratio with magnesium, so if you take 1000 mg calcium, take about 500 mg magnesium. The two minerals work together to control the heartbeat and an imbalance created by taking calcium alone can cause heart palpitations. Magnesium also has great value to thals.