It's about three-fifths as heavy as a Nail.
In other words, 491 miligrams is 0.62 times the weight of a Nail, and the weight of a Nail is 1.6 times that amount.(for 15-gauge, 2-inch, finishing nails)
It's about two times as heavy as a Raindrop.
In other words, the weight of a Raindrop is 0.5 times 491 miligrams.(a.k.a. drop, a.k.a. droplet) (average)
The weight of a raindrop depends heavily on the type of weather, but averages about 200 mg. The familiar teardrop shape of a raindrop is actually an optical illusion, caused by the reflection of the light and the motion of the drop.
It's about half as heavy as a Jelly Bean.
In other words, 491 miligrams is 0.45 times the weight of a Jelly Bean, and the weight of a Jelly Bean is 2.2 times that amount.(for Jelly Belly beans; approximate)
It's about one-third as heavy as a Paper Clip.
In other words, the weight of a Paper Clip is 2.9 times 491 miligrams.(average)
It's about one-fourth as heavy as a Playing Card.
In other words, 491 miligrams is 0.27 times the weight of a Playing Card, and the weight of a Playing Card is 3.7 times that amount.(ISO 216 B8 size, 2.5" x 3.5")
It's about one-fifth as heavy as a Penny.
In other words, the weight of a Penny is 5 times 491 miligrams.(United States) (formally one-cent coin) (Union shield design, 2010-present)
It's about one-tenth as heavy as a Die (Dice).
In other words, 491 miligrams is 0.12 times the weight of a Die (Dice), and the weight of a Die (Dice) is 8.3 times that amount.(for six-sided, 16 mm, rounded cube)