Could you please tell me how much a diamond the size of a baby's fist would be? I very much would like to know.
Infant hands are small but chubby, making them a bit closer to a sphere than a typical adult's hand. I'd peg them at very very roughly a 1.5 cm radius sphere, or about 14.1 cm^3. Since diamond has a density of about 3.51 g/cm^3, that's roughly 49.6 grams of diamond. Assuming the diamond weighed that much
after cutting, it would be approximately 248 carats. The Centenary Diamond is 273 carats, which is close enough for our purposes (we're working with gross approximations anyway). It's estimated to be worth about $100,000,000. (Note that this demonstrates the nonlinearity of carat weight to dollar value: the Greater Star of Africa isn't even twice as large, but best estimates put it at least four times as valuable as the Centenary Diamond.) I'd estimate the baby-fist-sized diamond at, say, $75,000,000 (roughly proportional to the squared ratio of their sizes).
So...yeah. Diamonds have incredible value, and the main reason people think of things like "fist-sized" gems is because Fantasy Authors Have No Sense Of Scale. In our world, good-quality diamonds the size of your thumbnail are worth hundreds of thousands of dollars.
For comparison, a gold piece in 2e (and by implication later editions) is roughly 9 grams of gold, such that there are about 50 gold pieces per (avoirdupois) pound of gold. 9 grams of gold is worth (very) roughly $500. So, if we use 1 gp = $500, a 5000-gp diamond is worth roughly $2,500,000. The baby's-fist-size diamond would be worth, again very roughly, 150,000 gp. The Greater Star of Africa, the single largest high-quality cut diamond extant today, would be worth very roughly 800,000 gp.
As stated, the sense of scale is so completely off, it's hilarious. A 5000 gp diamond, from what I can tell, would be about 10 carats. That's 2.0 grams, or 0.57 cubic centimeters (given its density of 3.51 g/cm^3). For comparison, a Jelly Belly bean is very roughly 2.6 cubic centimeters in volume, so a 5000 gp diamond would be in the ballpark of a quarter of a Jelly Belly bean.