Crystals can be "activated" by casting a spell on it, or by shining a light into it. In the confines of a Crystal Cave, however, this is extraordinarily dangerous, as light reflected from one crystal will activate any other crystals that it comes into contact to, with a combination of results and power. This will quickly cascade out of control until the amount of magical energy being simultaneously released is enough to kill an average human with radiation poisoning in a matter of seconds. The crystals range in power by color, with the following order from weakest (and largest) to strongest.
Blue: Often weighing upwards of several tons, blue crystals can aid in illusion magic. Bigger crystals can actually create random illusory effects, such as making the room its in appear upside down, etc.
Green: Green Crystals can heal damage and create minor plant life, such as plankton and grass. These crystals often weigh upwards of 1500 lbs, and are almost useless other then healing minor burns or wounds and growing lawns.
Purple: Purple crystals have the unusal property of being extremely attractive to vermin. Any type of insect, arachnid, rodent, etc will hoard as much purple crystal as possible into their lair. Oftentimes, giant spiders will duel over an average (about 1000 lb) chunk of crystal, with the winner and survivor claiming it. It is unknown what positive effect the crystal has on the vermin that possess it, if any.
Red: Red crystals, predictably, enhance fire. If used in a fire spell, the spell is maximized. If thrown into a burning fire, the fire rages out of control and burns hotter than almost any magical fire.
Orange: Orange crystals are effective at causing intense emotions in whoever is subjected to them. Often used in love potions, or drugs to inspire battle fury in Barbarians or Berserkers. They often weigh up to 700 lbs.
Yellow: Yellow crystals channel positive energy. They yield powerful undead destroying results, but have an unusual property of not healing wounds. These crystals are large, weighing 900 lbs on average, but grow in a lattice that can be broken off by hand.
White: White crystals cause massive explosions, which deal a mixture of cold and sound damage. Depending on the size of the crystal (typically, and counter intuitively) the smaller the crystal is, the more potent it is. The leading theory behind this is that, unlike the other colored crystals, white crystals have a static amount of power. The smaller the crystal is, the more compressed that power is. A typical white crystal (usually about 500 lbs) will, when activated, deal 5d6 points of cold damage and 3d8 points of sonic damage. Very rarely white crystals will only grow to a size of several inches. These crystals deal 15d6 points of cold damage, followed by 9d8 damage worth of sonic. The sonic damage cannot be overcome by any known ability, resistance, or immunity.
Golden: Golden crystals are almost immeasurably powerful against constructs. These crystals grow much differently then others do, and as such these come with a number of charges, much like the average staff. Once the charges are depleted, the Crystal will either break into a fine, golden-silvery dust, burn itself into nothing, dealing 1d6 points of fire damage to anything in a 5 foot radius, or simply vanish. Typically, a large gold crystal (about 25 lbs) will contain 10 charges. Destroying a construct uses 2 charges. Deactivating a construct uses 2 charges for permanent deactivation, or 1 charge for a temporary (24 hour) deactivation. You can however, spend 5 charges to gain control of a construct that obeys your commands indefinitely. Reactivating a deactivated or inert construct uses 1 charge.
Grey: A grey crystal controls sound. Typically weighing only about 5 lbs, a grey crystal will cause sounds to appear at the controllers will, with a moderate range. Almost any sort of sound can be replicated. This works in reverse, as sounds can be muted within a 25 foot radius of the crystal. Grey crystals can only be used for about 10 minutes each day, or else they "overheat" and suffer a chance of exploding. (This chance is 10%, plus 10% for every previous time the crystal has been used for upwards of 10 minutes.)
Silver: A Silver crystal looks almost identical to a grey, but has a much different MO. Whereas grey crystals control sound, silver control light. A silver crystal can create false images and projections, and can also bend light in ways that are typically impossible. This can make the user appear in a different area then he is, or even simply invisible. It suffers the same overuse penalty as grey, but the chance of explosion increases by 15% instead of 10.
Pink: Pink crystals control the most peculiar aspect of time. Pink crystals only grow to about 4 inches in length, and about 1 inch wide. While holding a pink crystal, twice per day, you may go back in time by 1 round. This takes back all actions and movements done in the previous round. This is a full round action.
Black: The black crystal is a legend, and does not exist. If it DID exist however, the fabled black crystal would not have the power over life, death, and necromancy as one might expect, but over power itself. The black crystal, if it was real, would command untold power for anyone who darted to hold it. Only one is rumored to exist, but it is simply that, a rumor.
Shouldn't this be in the homebrew forum?
In any event, this seems very 3.5-like, not 4e. Is that your intention?
Shouldn't this be in the homebrew forum?
In any event, this seems very 3.5-like, not 4e. Is that your intention?
Oh sorry, I havn't used the forums in awhile... Yeah thinking back it should probably be moved.
Well, yes and no? I tried to blend it so it was useable for both 4.e and 3.5, I guess i didn't do to great...
Dont worry too much about it people might find it usefull still 
. (was thinking of making magic crystal cave myself. just your version is tad too deadly). Try to PM some mod to get it moved or w8 untill one stumbles upon it.
Yeah, its cool. Could be used in either edition. Probably need a bit more mechanical tweaking for 4e, but good ideas are good ideas. Seems like I remember some other game at some point that had something really close to this, or maybe it was an old module...