# Relative weight of diamonds and rocks, for the purposes of disarming a trap (need answer fast) **URL:** \ **Category:** Factual Questions\ **Created:** [December 5, 2020, 2:02pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299 "2020-12-05T14:02:10Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![TwoCarrotSnowman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/twocarrotsnowman/32/15879_2.png) [@TwoCarrotSnowman](https://boards.straightdope.com/u/TwoCarrotSnowman)\ **Post date:** [December 5, 2020, 2:02pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/1 "2020-12-05T14:02:10Z")
Sadly, it’s not as exciting as the title makes out… I’m DMing an online Dungeons and Dragons session in about an hour. In a last-minute burst of creativity, I’ve come up with a trap that our brave heroes will encounter. It’s your standard Indiana Jones something-valuable-on-a-weighted-platform thing - in this case, a diamond as big as your fist. If the adventurers pick up the diamond, the platform moves and… well, you get the idea. I’m pretty certain the players will come up with the idea of picking up a rock from the rubble in the room and trying to swap it with the diamond (much like Indy does in the movie). I don’t have any sense of how big a rock it would need to be in order to weigh roughly the same as the diamond - the same size? Much bigger? Much smaller? The idea would be that if the players use a rock that’s either too heavy or too light, then… kaboom, but in order to do that I need a reasonable idea of how big a rock they should use. Rough order of magnitude is all I need. Posted in GQ because although it’s about tormenting a party of D&D players, it’s basically a geology question. 😀 ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [December 5, 2020, 2:08pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/2 "2020-12-05T14:08:55Z")
Quick Googling. Diamond is 3.51g/cubic cm, granite is 2.75g/cubic cm. ---
**Author:** ![What\_Exit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/what_exit/32/10652_2.png) [@What\_Exit](https://boards.straightdope.com/u/What_Exit)\ **Post date:** [December 5, 2020, 2:10pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/3 "2020-12-05T14:10:51Z")
Diamonds have an average density of about 3.3g/cm3 while Quartz is about 2.65g/cm3. Limestone is about 2.7g/cm3. So the Diamond part isn’t too hard, but estimating the density of the rock can vary. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 5, 2020, 2:15pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/4 "2020-12-05T14:15:42Z")
Diamonds are denser than random bits of gravel, but not twice as heavy. As for how it works in practice, you’ve seen _Raiders of the Lost Ark_ so you know the exact method 🙂 (you don’t pick just one rock, not that it ultimately matters…) ---
**Author:** ![TwoCarrotSnowman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/twocarrotsnowman/32/15879_2.png) [@TwoCarrotSnowman](https://boards.straightdope.com/u/TwoCarrotSnowman)\ **Post date:** [December 5, 2020, 2:23pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/5 "2020-12-05T14:23:09Z")
Thanks everyone. For the purposes of the game, I’m going to say that a rock on the platform needs to be 1.2 times the size of the diamond (or be made up of smaller rocks that add up to the same thing) - or in gameplay terms “bigger, but not much bigger”. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 5, 2020, 2:27pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/6 "2020-12-05T14:27:31Z")
Is there anyone who, on seeing the title, didn’t think “This must be for a D&D game”? And then further anticipate the exact situation? And if there’s even the slightest hint that they might get the weight wrong and trigger the trap, they’re going to instead come up with some elaborate plan involving ten-foot poles, three adventurers’ supply of rope, a Mage Hand, and a bobby pin. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [December 6, 2020, 7:36am UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/7 "2020-12-06T07:36:22Z")
Or a **Telekinesis** spell from (hopefully) far enough away. Unless there is a **Dispel Magic** in effect … ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [December 6, 2020, 3:37pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/8 "2020-12-06T15:37:32Z")
> [@TwoCarrotSnowman](#): > > For the purposes of the game, I’m going to say that a rock on the platform needs to be 1.2 times the size of the diamond Size may be ambiguous. It’s volume, so a touch lower if diameter. Depending on how fussy you want to be here. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 6, 2020, 6:29pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/9 "2020-12-06T18:29:00Z")
> [@Ruken](#): > > Size may be ambiguous. It’s volume, so a touch lower if diameter. Depending on how fussy you want to be here. It’s a D&D game. You don’t want to get too fussy, since that detracts from the fun of playing. Most D&D rules that aren’t about magic are simplifications of reality to make the game flow better. The rule TwoCarrotSnowman came up with is about as fussy as you want to get. Anything more complicated gets the game bogged down in unnecessary details. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [December 6, 2020, 8:28pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/10 "2020-12-06T20:28:51Z")
Around 1.2 x is a good answer. But as a DM, I’d only allow that kind of knowledge to someone with a mining or gemcutting NWP roll or a similar background check. Everyone else is going to get it wrong enough to set off the trap - relative densities is a pretty sophisticated subject for most characters. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 6, 2020, 9:05pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/11 "2020-12-06T21:05:40Z")
And you want to make it dependent on the characters’ abilities, not on the players’. The player is probably going to say something like “I look around for a rock that looks like it weighs about as much as the diamond”. And then how close the weight actually is will depend on how familiar they are with diamonds, how good they are at estimating weights, and how experienced they are at interacting with traps. They’re not going to say “The diamond is 7.1 cm in diameter, so I want a rock that’s 8.6 cm in diameter”. Just pick a skill, and set a DC. Or maybe set two DCs, to allow for a case where they didn’t make the swap but also didn’t set off the trap (they started to swap, but realized that the pressure plate was moving slightly as they did, so stopped before it moved enough to trigger). ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [December 6, 2020, 9:09pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/12 "2020-12-06T21:09:59Z")
This While there is some RP to be done here, it should really be based on an intelligence roll or a disarm traps roll to see how accurately the character estimated the weights. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 6, 2020, 10:31pm UTC](https://boards.straightdope.com/t/relative-weight-of-diamonds-and-rocks-for-the-purposes-of-disarming-a-trap-need-answer-fast/927299/13 "2020-12-06T22:31:56Z")
> [@Chronos](#): > > They’re not going to say “The diamond is 7.1 cm in diameter, so I want a rock that’s 8.6 cm in diameter”. Of course they wouldn’t. That would make the rock 78% heavier, not 21%.