Diamond Mine is a fairly tough mode as it's set against a timer and can involve some difficulty if there are no potential matches near the rock at the bottom.
Your best bet is to keep focusing down by the rock. Any matches you can make there will help you progress. Failing that, try to set up a special gem.
Fire gems - get a four in a row match for these explosive gems. They aren't amazing but they can clear out a few gems, giving you a chance of making some matches with what drops in.
Hypercubes - get five in a row for one of these. Look at the rock below and work out which colour gem is likely to do the most damage to it and then push your hypercube into a gem of that colour anywhere on the board. These are very handy.
Star gems - get an L or T shape with five gems for one of these. They wipe out an entire row and column. If you have one of these down below the rock level it can get you an instant depth drop. The most useful gems in the game.
Also: don't aim for 200. Aim to keep going until you get that AND unearth 12 relics (these are randomly placed and unmissable) as long as you get down to their level and past it.