A large red crystal is a rare, naturally-generated resource that looks like a long red crystal protruding vertically from a clumpy base of smaller red crystals. This item stands two blocks high and one block wide, and has full collision, except it cannot contain liquids. You can find this item in the Resources section of inventory.
How to Obtain
The only way to obtain large red crystals is either to mine them in hell or deep biomes (the only two biomes where they generate naturally in underground caves) or to trade for them in a market world.
How to Use
Unlike small red crystals, large red crystals are not an ingredient in any crafting recipe. Their end use is often decorative, but due to their rarity, they sometimes serve as a standard of currency in trading as well.
Large red crystals were first introduced in Spring of 2014, and for the first year or so they had unique collision features: they would stop mobs but not players or projectiles. Many structures and building competition entries from that era used them as the background decorative items that they seemed to be, as openings to mob farms (since they would keep mobs in but allow player gunfire through), or as clever doorways to pet or android enclosures (since they would keep pets in but allow players to pass through).
Sometime in mid-2015, this collision was changed so that large crystals of all types had the same collision as small ones. As a result, many structures that had used them as decorative background items became suddenly impassable. Sadly, some of these structures are in competition worlds, where they are permanently preserved in their un-navigable states, since once the building phase of a competition ends, the competition worlds lock down and players are unable to go back and make any changes.