Obsidian based rock? The vinegar test isn't as accurate. There is some sort of acid (can't remember which) you can buy that is better.
Also:
How to Test
Testing can be as simple as placing a few drops of vinegar on the rock or gravel you are considering using. If it fizzes or foams, don't use it, as it contains calcium deposits. Another way of testing rocks and gravel is to place them in a bucket of the same water that you use in your aquarium. Test the pH and hardness, and then let it sit for a week and test again. If there is a significant change, the rocks or gravel are likely to cause problems.