Most trapiche stones are Colombian emeralds (see http://on.fb.me/1GHHKnQ), though other minerals such as iolite (aka cordierite, or in this incarnation cherry blossom stone, see http://on.fb.me/1RJiJsF) also crystallise in this unusual configuration. Corundum (both sapphire and ruby) also produces the occasional specimen like this lovely hundred carat beauty from Burma seen here (measuring 35.47 x 31.89 x 6.70mm). As you can observe, the crystal has grown in a radial manner around an older grey corundum core, with the iron and titanium that pass an electron back and forth to create the blue colour being selectively distributed and separated by bands of colourless to light yellowish corundum.
These stones are not to be confused with star sapphires or rubies (see http://on.fb.me/1PCk2bTand http://on.fb.me/1EoGpBi), which are due to light being reflected off fine needle shaped inclusions in the stone, also orientated following the mineral's hexagonal crystal structure. Whether caused by inclusions or colour zoning, these stones reveal well how crystals grow and the invisible rules of their formation that causes atoms to organise themselves in regular repeating lattices. Water crystals grow in this system too, and the diverse shapes of snowflakes are another example of this crystalline structure revealed.
Image credit: Bonhams No automatic alt text available.