Here's one I made earlier

 

I've been attempting to grow my own calcite.  The reason for doing this is to create whats known as a standard. We dope the calcite with chemicals we want to analyse in other things. Then when we know what the concentration of these chemicals (or elements) are in the calcite (which we get from using other machines) we know how the machine we are measuring our samples on is behaving.  As a result we can quantify our results rather than just say that the machine measures this much of an element. For most things you can buy standards from various companies.  But for some elements in a calcite matrix (the background structure) there is no standard, so you have to make one yourself.

Its been a tricky procedure full of lots of chemistry (remember doing titrations at school, well I was doing them for real - as a scientist - wow!).  Though it has been good fun. It basically involves mixing calcium hydroxide with carbonic acid (which you make by bubbling carbon dioxide through water), plus whatever you want to dope the calcite with.  Simple enough in theory, but when you have to make/mix your own chemicals and then dilute them to the correct concentrations it actually ends up taking all day to do, plus a day beforehand to get hold of all the equipment. And even then it didn't really go too well, no calcite was growing in my beakers.

So the following morning I checked the pH of my solution and found it was far too high, so I threw quantitative chemistry out the window and bubbled more carbon dioxide through the solution until the pH lowered.  And low and behold, after a few more hours back on the stirring plate, calcite was beginning to grow.  Hooray.